Develop Bugs

Can H811 Bootloader Be Re-Locked?

I'm stuck and confused:
Had a Nexus 4 and 5 for years. Decided to upgrade this past November to the LG G4 (T-Mobile H811)
Got it to root without touching the boot loader.
In came MM and then I had issues of repeating the process.
I came across a solution, to unlock my bootloader. So I did it and managed to get my G4 rooted while running stock LG MM (H81120i).
Everything was running well for several months, but then I started to see erratic behavior. It would heat-up in my pocket for no reason. It would not save pictures. Connections via WiFi and LTE were breaking up. All in all, I wasn't happy and I missed the stock Google interface.
So I've decided to install CM13. Had many issues of getting it to work for several days, which included persistent closings of Google Play services (and many others) and not being able to make/receive calls (it would automatically reboot) After several different attempts, I managed to get it all working. Cleared/Wiped everything off my internal drive, installed the "LG-H81120i-Flashable.COMPLETE" and "cm-13.0-201605xx-NIGHTLY-h811" and "open_gapps-arm64-6.0-nano-201605xx", all back to back. And then installed manual everything as needed Gmail, Maps, etc.
Everything worked great for about 2-days, until then next morning my screen locked up. Then the phone rebooted. After the reboot, it lasted for about 10-15 seconds in CM13 and rebooted again manually. Continued to reboot over and over, till it then stopped booting with CM13. I thought that maybe I needed to install the latest CM13 zip file, but found I couldn't get into recovery. The first reboot screen, showing "Bootloader State: Bootloader unlocked? and the LG logo would be in a loop. During that time, I could feel the phone getting hotter and hotter.
Sometimes, the phone would boot up into CM fully and last for 5 to 10 minutes. Then reboot again and stay in the first reboot screen.
I managed to get into TWRP recovery and thought that putting back the stock MRA58K image would help. During the install in recovery, the phone rebooted! I never thought it could do that. Eventually I got the stock LG back-up up and running, but it didn’t help, and same rebooting occurred.
I'm stuck and out of options. I would like to get the phone BACK TO STOCK and then perform an RMA. I’m afraid that with the BOOTLOADER UNLOCKED that my RMA will be denied and I’m stuck with what I have.
So can the H811's bootloader be re-locked?

PiCASSiMO said:
I'm stuck and confused:
Had a Nexus 4 and 5 for years. Decided to upgrade this past November to the LG G4 (T-Mobile H811)
Got it to root without touching the boot loader.
In came MM and then I had issues of repeating the process.
I came across a solution, to unlock my bootloader. So I did it and managed to get my G4 rooted while running stock LG MM (H81120i).
Everything was running well for several months, but then I started to see erratic behavior. It would heat-up in my pocket for no reason. It would not save pictures. Connections via WiFi and LTE were breaking up. All in all, I wasn't happy and I missed the stock Google interface.
So I've decided to install CM13. Had many issues of getting it to work for several days, which included persistent closings of Google Play services (and many others) and not being able to make/receive calls (it would automatically reboot) After several different attempts, I managed to get it all working. Cleared/Wiped everything off my internal drive, installed the "LG-H81120i-Flashable.COMPLETE" and "cm-13.0-201605xx-NIGHTLY-h811" and "open_gapps-arm64-6.0-nano-201605xx", all back to back. And then installed manual everything as needed Gmail, Maps, etc.
Everything worked great for about 2-days, until then next morning my screen locked up. Then the phone rebooted. After the reboot, it lasted for about 10-15 seconds in CM13 and rebooted again manually. Continued to reboot over and over, till it then stopped booting with CM13. I thought that maybe I needed to install the latest CM13 zip file, but found I couldn't get into recovery. The first reboot screen, showing "Bootloader State: Bootloader unlocked? and the LG logo would be in a loop. During that time, I could feel the phone getting hotter and hotter.
Sometimes, the phone would boot up into CM fully and last for 5 to 10 minutes. Then reboot again and stay in the first reboot screen.
I managed to get into TWRP recovery and thought that putting back the stock MRA58K image would help. During the install in recovery, the phone rebooted! I never thought it could do that. Eventually I got the stock LG back-up up and running, but it didn’t help, and same rebooting occurred.
I'm stuck and out of options. I would like to get the phone BACK TO STOCK and then perform an RMA. I’m afraid that with the BOOTLOADER UNLOCKED that my RMA will be denied and I’m stuck with what I have.
So can the H811's bootloader be re-locked?
Click to expand...
Click to collapse
Since you live in the US your only option is to apply over voltage to the battery terminal and burn the mobo so that it wont even boot up. The exact method is somewhere around here in xda so you should look it up since you have no other choice
Edit: To answer your question: No

sirsomething said:
Since you live in the US your only option is to apply over voltage to the battery terminal and burn the mobo so that it wont even boot up. The exact method is somewhere around here in xda so you should look it up since you have no other choice
Edit: To answer your question: No
Click to expand...
Click to collapse
If I can't re-lock the bootloader, has anyone had the success of returning their phones under warranty? Through T-Mobile?

Interrupted flashing during "VolumeUp + Power" mode

I have updated my Nexus 5x to 7.1.1 beta and after a day it powered off and I got the infamous red led during charging. I was not able to turn the phone on even after leaving the charger on for some days. I thought it is bricked and left it alone connected to my computer.
However today, out of nowhere Google logo popped up. It was bootlooping, but I was able to enter bootloader and original recovery. I wanted to flash it from bootloader, but was unable, because it was still oem locked. And I was not able to oem unlock, because I did not do the required part in the Developer Tools menu. I tried to adb sideload OTA in the original recovery, but sideloading stopped after uploading ~48% every time.
So I opted for the "VolumeUp + Power" mode flashing. Phone was recognized and I started flashing the appropriate TOT file. However at 33% on phone and 28% on the PC, progress stopped. And after a minute or so phone turned off. Currently it has no signs of life at all. Have I completely bricked it now? Is there a way to recover from interrupted "VolumeUp + Power" flashing mode?
EDIT: at first I though maybe the battery died during flashing. But interruption was not abrupt. First the progress slowed down to halt. And then after some time phone turned off.

It seems not all is lost. Have a blinking red light again after leaving it connected to computer for a day. Back to square one, but at least it is alive.

Your motherboard is dead.
And it is not your fault, mine also died while using beta 7.1 and I sent it RMA. They replaced for a new one.
But in my case i had unlocked bootloader, I flashed back 7.0 and it gave no results, still bootloop that's why I sent

Thanks for the info.
After leaving my phone with a clamp on a power button throughout the night (so the red light would flash continuously), I got it booting into "VolumeUp + Power" mode and this time I have successfully flashed the same TOT file via LGUp software. It was still bootlooping after flashing. Now I was able to enter into bootloader and recovery and this time sideloading the same previous file actually works. However it still bootloops.
Anyway, I tried to RMA it and after giving my IMEI to the Google rep, they said that they could not find it in their system after checking US and UK store records. Also tried calling LG warranty, but they do not know of my IMEI either. I got this phone from Swappa, so I left wondering where did the original owner of this phone got it from. He claimed that he got it as a present...

BlinK_ said:
Thanks for the info.
After leaving my phone with a clamp on a power button throughout the night (so the red light would flash continuously), I got it booting into "VolumeUp + Power" mode and this time I have successfully flashed the same TOT file via LGUp software. It was still bootlooping after flashing. Now I was able to enter into bootloader and recovery and this time sideloading the same previous file actually works. However it still bootloops.
Click to expand...
Click to collapse
out of curiosity, have you ever tried to flash custom recovery through the bootloader ? If you can, you might also wanna try to make a EFS backup, wipe all partition, then flash custom rom or near-stock rom like this one, just for the sake of testing.

anazhd said:
out of curiosity, have you ever tried to flash custom recovery through the bootloader ? If you can, you might also wanna try to make a EFS backup, wipe all partition, then flash custom rom or near-stock rom like this one, just for the sake of testing.
Click to expand...
Click to collapse
I wanted to flash custom recovery but was unable to do so, because device is still OEM locked. And I am unable to unlock it, because I did not do the confirmation in the Developer Options before device started to bootloop. Is there a way to flash custom recovery without this step in Developer Options?

BlinK_ said:
I wanted to flash custom recovery but was unable to do so, because device is still OEM locked. And I am unable to unlock it, because I did not do the confirmation in the Developer Options before device started to bootloop. Is there a way to flash custom recovery without this step in Developer Options?
Click to expand...
Click to collapse
For now, no way that I can think of. You could try to oem unlock it until it work, just cross your finger I guess?

I have some progress here. Since I was able to sideload various OTA images, I started doing that one by one and seeing if some of them would not bootloop. And after flashing 7.0.0 (NRD91N) from here (https://developers.google.com/android/ota) phone booted successfully and now is working. Will report if I get any problems.

HTC One M9 Stuck in HTC logo. Bootloop in download mode. S-On, Locked, Official Rom

HTC One M9, no branding, official software, locked and S-ON.
one month ago my phone got (for no apparent reason) serious software problems, so spend a lot of time on the internet to find a solution. in the end, i downloaded the correct stock rom, put it on a sd card and reinstalled stock android via the download mode. it worked out really well, my phone was running like new.
but 2 days ago i pick up my phone and realize it's stuck in a bootloop. (again for no apparent reason) using the right button combinations, i am able to get into the bootloader.
when i try to flash the stock rom with the exact same method that worked last time, it says "start ui_updating" and gets caught in the bootloop again. (i dont think i did anything wrong because the exact same procedure worked last time)
when i enter recovery mode, i get the infamous red triangle. using the right buttons, i can still access the recovery mode from there. now, when i try to factory reset from here, i get the following errors: E: failed to mount /preload (invalid argument).. try emmc mount E: Cant find device node for mount point again E: mount /cache fail, format it and mount again ----- about 5 more similar errors after those 3.
I tried to flash recovery img with fastboot but i get an error similar to:
9 RU_SECURITY_FAIL recovery and bootloader isn't BL_UNLOCKE
I guess this is because my phone doesn't have usb debugging enabled or it doesn't have an unlocked bootloader.
I also tried to unlock the bootloader but after running the command: fastboot oem get_identifier_token i don't get the token so i can post it to htcdev and unlock my bootloader.
Anybody help or had the same issue and fixed it?
Thanks in advance

DE2IS said:
HTC One M9, no branding, official software, locked and S-ON.
one month ago my phone got (for no apparent reason) serious software problems, so spend a lot of time on the internet to find a solution. in the end, i downloaded the correct stock rom, put it on a sd card and reinstalled stock android via the download mode. it worked out really well, my phone was running like new.
but 2 days ago i pick up my phone and realize it's stuck in a bootloop. (again for no apparent reason) using the right button combinations, i am able to get into the bootloader.
when i try to flash the stock rom with the exact same method that worked last time, it says "start ui_updating" and gets caught in the bootloop again. (i dont think i did anything wrong because the exact same procedure worked last time)
when i enter recovery mode, i get the infamous red triangle. using the right buttons, i can still access the recovery mode from there. now, when i try to factory reset from here, i get the following errors: E: failed to mount /preload (invalid argument).. try emmc mount E: Cant find device node for mount point again E: mount /cache fail, format it and mount again ----- about 5 more similar errors after those 3.
I tried to flash recovery img with fastboot but i get an error similar to:
9 RU_SECURITY_FAIL recovery and bootloader isn't BL_UNLOCKE
I guess this is because my phone doesn't have usb debugging enabled or it doesn't have an unlocked bootloader.
I also tried to unlock the bootloader but after running the command: fastboot oem get_identifier_token i don't get the token so i can post it to htcdev and unlock my bootloader.
Anybody help or had the same issue and fixed it?
Thanks in advance
Click to expand...
Click to collapse
Hi Sorry to say, I have " Exactly" the same problem and have also reached the same place, I asked on these forums without success. in the end I sent it back to HTC and they want £200 to repair it or £29.00 diagnostic fee. I have asked them to send it back and will have to pay the £29.00. If it is within warrantee send it back.

m01y1i00 said:
Hi Sorry to say, I have " Exactly" the same problem and have also reached the same place, I asked on these forums without success. in the end I sent it back to HTC and they want £200 to repair it or £29.00 diagnostic fee. I have asked them to send it back and will have to pay the £29.00. If it is within warrantee send it back.
Click to expand...
Click to collapse
Thanks for your fast reply.
I will get in contact with them and see what they will say. My phone is under warranty so i will pay only for shipping charges.
I bought it from a german eshop and i am from Cyprus.
The problem is, the official distributor for HTC here in Cyprus is Greece and i had a bad experience with them. I sent an old HTC One X 4 years before which had a board issue, and after waiting for a month they sent it back and guess what. It had the same problem. So i got in contact with the US website, described the problem and mentioned also the "official distributors" in Cyprus and Greece and how their "service" was and they told me i will have a brand new phone here in Cyprus within a week (which became a reality).
So before i experience the same issues i will clear everything from here (if anybody know a successful solution) and then communicate with HTC how to resolve this.
Thanks again for your reply!

Is this a general/common problem on htc? I am worried

Spike97 said:
Is this a general/common problem on htc? I am worried
Click to expand...
Click to collapse
I'd be lying if I'd say that I haven't read about the issue several times, already. However, you need to be aware of the fact that most of the time only people with problems are creating threads whereas people who have no issues at all won't do the same for praising the phone.
In other words: Yes, this problem does exist but no, not everyone faces it. I know many users whose M9 is still working fine.

DE2IS said:
HTC One M9, no branding, official software, locked and S-ON.
one month ago my phone got (for no apparent reason) serious software problems, so spend a lot of time on the internet to find a solution. in the end, i downloaded the correct stock rom, put it on a sd card and reinstalled stock android via the download mode. it worked out really well, my phone was running like new.
Click to expand...
Click to collapse
I also have a bootloop issue with Locked and S-ON... Care to let me know how you managed to get it working the 1st time?
Where to find the corrent stock ROM and the method you did to get it working..

i have the same problem. Where can i get a rom.zip

Bootloop
gmayer1212 said:
i have the same problem. Where can i get a rom.zip
Click to expand...
Click to collapse
Take the new OTA Nougat, separate those three files on your sd card
Install it with fastboot
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash system system.img
and it will be working takes 5-10 minutes to restart

kwaichang said:
Take the new OTA Nougat, separate those three files on your sd card
Install it with fastboot
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot flash system system.img
and it will be working takes 5-10 minutes to restart
Click to expand...
Click to collapse
Please don't give such advice. If the firmware isn't matching the system version users will face bugs and lags or the phone won't even boot. (The latter happens for example if you try to use an Android M system with an Android L firmware.) Besides a user with a locked bootloader and a firmware/system-mismatch won't be able to install any OTAs unless that person flashes a RUU for fixing the mismatch*. The catch is there is no publicly available Android N RUU for every variant of the M9, at the moment. And with S-ON you aren't able to use older RUUs. That means people might have to contact Llabtoofer's RUU Service (which isn't for free!) if they followed your recommendation.
* Users with an unlocked bootloader need to fix the mismatch, as well, but at least they have twrp backups as an alternative solution for the problem.
@gmayer1212 & @htcm9guy: You may want to read the ReadMe thread. It's pinned above the normal threads here in this section.

Flippy498 said:
Please don't give such advice. If the firmware isn't matching the system version users will face bugs and lags or the phone won't even boot. (The latter happens for example if you try to use an Android M system with an Android L firmware.) Besides a user with a firmware/system-mismatch won't be able to install any OTAs unless that persons flashes a RUU for fixing the mismatch. The catch is there is no publicly available Android N RUU for every variant of the M9, at the moment. And with S-ON you aren't able to use older RUUs. That means people might have to contact Llabtoofer's RUU Service (which isn't for free!) if they followed your recommendation.
@gmayer1212 & @htcm9guy: You may want to read the ReadMe thread. It's pinned above the normal threads here in this section.
Click to expand...
Click to collapse
Sorry for that. Mine was also hanging in bootloop but unlocked S-on T-mobile Germany

kwaichang said:
Sorry for that. Mine was also hanging in bootloop but unlocked S-on T-mobile Germany
Click to expand...
Click to collapse
Don't get me wrong, I don't want to discourage you from helping others. It's just important that you know which variant of the M9 the user with the problems owns in such cases. Just recommending to flash random stock files only causes more problems than it solves.

Just to chime in --- same problem happened to me yesterday.
Phone has been working fine --- then yesterday phone went unstable so I reset and got caught in bootloop.
After doing a factory reset and many tries -- got it to boot to android, only to get bombarded by google play service errors -- most likely wrong version of google play services for android?? I manually installed google play services apk... Reset and after many tries was able to boot again -- system was smooth. But did a quick reset to check whether problem was fixed and now i'm back in boot loop.
I wonder if there's a problem with a recent google apps update over playstore that's killing boot?

Hello,
i have the same problem but in some ways different. My M9 was running nearly fine until yester der evening. I just plugged the power supply and suddenly black screen, bootloop. But the really big problem is i don't come in any different stable mode than bootloader. If i try to get into download or recovery mode, the silver htc-logo appears for less than a secound, short vibration and reboot - and this is in loop also.
So for me i don't se a way to factory reset or reinstall any RUU or anything else?
Is it destroyed? :crying:
I was complete S-Off before, running on the last firmware with ViperOneM9 6.1.0.
Thans for your support,
Bond

That sounds like the dead nand issue. It's a hardware failure (c.f. the FAQ of the ReadMe thread).
Sent from my HTC One M9 using XDA Labs

If your phone is under warranty, it has a stock rom, and bootloader is still locked, then send it to HTC after contacting them via chat.
They will replace either the board or the device.
This is whhat they did for me.

DE2IS said:
HTC One M9, no branding, official software, locked and S-ON.
one month ago my phone got (for no apparent reason) serious software problems, so spend a lot of time on the internet to find a solution. in the end, i downloaded the correct stock rom, put it on a sd card and reinstalled stock android via the download mode. it worked out really well, my phone was running like new.
but 2 days ago i pick up my phone and realize it's stuck in a bootloop. (again for no apparent reason) using the right button combinations, i am able to get into the bootloader.
when i try to flash the stock rom with the exact same method that worked last time, it says "start ui_updating" and gets caught in the bootloop again. (i dont think i did anything wrong because the exact same procedure worked last time)
when i enter recovery mode, i get the infamous red triangle. using the right buttons, i can still access the recovery mode from there. now, when i try to factory reset from here, i get the following errors: E: failed to mount /preload (invalid argument).. try emmc mount E: Cant find device node for mount point again E: mount /cache fail, format it and mount again ----- about 5 more similar errors after those 3.
I tried to flash recovery img with fastboot but i get an error similar to:
9 RU_SECURITY_FAIL recovery and bootloader isn't BL_UNLOCKE
I guess this is because my phone doesn't have usb debugging enabled or it doesn't have an unlocked bootloader.
I also tried to unlock the bootloader but after running the command: fastboot oem get_identifier_token i don't get the token so i can post it to htcdev and unlock my bootloader.
Anybody help or had the same issue and fixed it?
Thanks in advance
Click to expand...
Click to collapse
is your phone recongized - what shows "fastboot devices"

pear-i said:
Just to chime in --- same problem happened to me yesterday.
Phone has been working fine --- then yesterday phone went unstable so I reset and got caught in bootloop.
After doing a factory reset and many tries -- got it to boot to android, only to get bombarded by google play service errors -- most likely wrong version of google play services for android?? I manually installed google play services apk... Reset and after many tries was able to boot again -- system was smooth. But did a quick reset to check whether problem was fixed and now i'm back in boot loop.
I wonder if there's a problem with a recent google apps update over playstore that's killing boot?
Click to expand...
Click to collapse
I too wonder the same - recently had the bootloop issue with my M9, just when it was updating apps from the Playstore... quite suspicious...

My HTC M9 is also in boot loop
htcm9guy said:
I also have a bootloop issue with Locked and S-ON... Care to let me know how you managed to get it working the 1st time?
Where to find the corrent stock ROM and the method you did to get it working..
Click to expand...
Click to collapse
It happened at last Saturday, when I've just tried to perform a regular reset to my HTC M9 (from the regular device's menu).
I can't enter to download mode (fail), wipe cache and wipe all (factory reset) didn't help.
Is there any chance to fix it at home? Or just to buy a new phone?
And, does this issue can occur in other devices, HTC or any other brand?
Please advise.
(Can't upload a picture of my device, since I'm a new user...)
Thanks!

As far as I know there's nothing you can do. There are no reports of successful diy repairs of a dead nand M9.
Theoretically this issue can happen with any phone. However, it looks like the probability of its occurence on a M9 increased a lot during the last months.

I´m also stuck in bootloader mode
recovery mode / download mode endup with red text:
"failed to boot to download mode"
***Software status: Modified***
***LOCKED***
***S-ON***
***Security Warning***
Cannot get the token as fastboot oem commands end up empty or unknown command.
I think USB-debbugging is "off"
How can i get S-OFF
or how can i get token
Thanks.

"Can't find valid operating system. The device will not start."

My wife got this error today:
"Can't find valid operating system. The device will not start."
Then shows the visit g.co/ABH link.
We aren't even able to boot into recovery. When selecting recovery from fastboot mode it just goes right to the same "Can't find valid operating system..." screen.
I've seen a few posts of people having this issue, I haven't seen anyone state they recovered, but I wanted to make sure.
I figure since I can't even boot to recovery there is nothing to be done??
History:
This phone is < 3 months old. It has not been modified (no unlocked bootloader, root, or even USB debugging enabled AFAIK). It rebooted on her once in the middle of her just using it within the past 48 hours. Today she saw a notification saying the phone needed to reboot because of a system update. She did not reboot it yet, but was using it fine. When she went to use it this afternoon (after having been on it 15 minutes prior) it was booted to this error screen.
On a side note, Google refuses to send a new phone even though this one is < 3 months old and the problem appears to have been caused by their software updating and rebooting her phone without permission.
thanks

TraderJack said:
My wife got this error today:
"Can't find valid operating system. The device will not start."
Then shows the visit g.co/ABH link.
We aren't even able to boot into recovery. When selecting recovery from fastboot mode it just goes right to the same "Can't find valid operating system..." screen.
I've seen a few posts of people having this issue, I haven't seen anyone state they recovered, but I wanted to make sure.
I figure since I can't even boot to recovery there is nothing to be done??
History:
This phone is < 3 months old. It has not been modified (no unlocked bootloader, root, or even USB debugging enabled AFAIK). It rebooted on her once in the middle of her just using it within the past 48 hours. Today she saw a notification saying the phone needed to reboot because of a system update. She did not reboot it yet, but was using it fine. When she went to use it this afternoon (after having been on it 15 minutes prior) it was booted to this error screen.
On a side note, Google refuses to send a new phone even though this one is < 3 months old and the problem appears to have been caused by their software updating and rebooting her phone without permission.
thanks
Click to expand...
Click to collapse
It is under warranty. Phones brick on updates more than any other time. I don't see where they really have a choice but to replace it.

TraderJack said:
My wife got this error today:
"Can't find valid operating system. The device will not start."
Then shows the visit g.co/ABH link.
We aren't even able to boot into recovery. When selecting recovery from fastboot mode it just goes right to the same "Can't find valid operating system..." screen.
I've seen a few posts of people having this issue, I haven't seen anyone state they recovered, but I wanted to make sure.
I figure since I can't even boot to recovery there is nothing to be done??
History:
This phone is < 3 months old. It has not been modified (no unlocked bootloader, root, or even USB debugging enabled AFAIK). It rebooted on her once in the middle of her just using it within the past 48 hours. Today she saw a notification saying the phone needed to reboot because of a system update. She did not reboot it yet, but was using it fine. When she went to use it this afternoon (after having been on it 15 minutes prior) it was booted to this error screen.
On a side note, Google refuses to send a new phone even though this one is < 3 months old and the problem appears to have been caused by their software updating and rebooting her phone without permission.
thanks
Click to expand...
Click to collapse
Two questions. 1) Was it purchased from Google? If not, that may be why the issue with warranty replacement. 2) Is the phone a Google version (AKA unlockable bootloader)? If it is, you could always do a bootloader unlock which will also force a factory reset and if successful allow you to load a factory image. If not, there is nothing you can do without getting into recovery.

5T - device corrupt

Hi Everyone,
I have a 5T running android 9 - completely stock. The device is SIM free, I have never customised it at all - if anything it's never been connected to a PC even.
A few days ago I was using my phone as normal (on whatsapp) but it completely froze. It now boots up to the "Your device is corrupt and cannot be trusted and may not work properly" - it freezes on start up and won't work at all.
- I can get to the fastboot screen
- I can get to the recovery screen to install from ADB
The fastboot screen says in red - device state - locked.
The device is a clean, un-customised phone. Only a few days prior I checked for any OTA updates but I haven't received android 10 yet.
I haven't flashed phone firmware or anything for years now. (By years, I literally mean years - back in the Nokia 8210 days). My only other experience of flashing firmware was with Samsung phones and 'debranding' them.
As I was not trying to root the device or anything - how do I fix this issue? I've searched the forums but I cannot fine a scenario similar to mine therefore I'm a bit confused as to what to do to resolve it.
I've tried to sideload this - oneplus.com/uk/support/softwareupgrade/details?code=PM1574156155944 via recovery but it says installation failed.
Any help appreciated thanks!

Sounds like you're straight out of luck to recover any data if your device is completely stock, as to unlock your bootloader to access encrypted data would mean a factory reset. You could try try one of the unbrick tools using the guide here. You will lose your data.

I tried the unbrick tool here - forum.xda-developers.com/oneplus-5t/how-to/oneplus-5t-unbricking-tool-confirmation-t3733012
... and it doesn't work. The phone now boots to a blank screen with a constant white LED.
Could this be a hardware issue?
I'm not too concerned about losing my data - already gave up on trying to keep my data when the problem first occurred!

After the 4th use of the unbrick tool (each time it completes successfully) I was finally able to boot into the phone.
I got as far as setting the region but then it wouldn't find any wifi networks. It then completely froze on this screen and is now back to "your device is corrupt and cannot be trusted" and will continually try to boot and freeze on the oneplus logo. Sometimes it will boot to a blank screen with a white LED.
Has anyone else had this issue? I'm definitely suspecting a hardware issue now...

oneplus5t_ said:
After the 4th use of the unbrick tool (each time it completes successfully) I was finally able to boot into the phone.
I got as far as setting the region but then it wouldn't find any wifi networks. It then completely froze on this screen and is now back to "your device is corrupt and cannot be trusted" and will continually try to boot and freeze on the oneplus logo. Sometimes it will boot to a blank screen with a white LED.
Has anyone else had this issue? I'm definitely suspecting a hardware issue now...
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Problem fixed?

unfortunately not. Any ideas on next steps?

That's an odd issue for sure. My only idea would be installing TWRP and maybe formatting from there and using fix permissions etc. to see if it has any effect.

I've done some googling to try and understand "TWRP" but I'm not sure I fully understand. Are there any forum posts I can refer to?
- phone has been flashed several times
- before this whole saga dev mode was enabled, but only to reduce animation scale - USB debugging was never enabled
- the phone has always been stock / sim free - never rooted
Thanks

Can you get into recovery? If yes then try connecting to a pc whilst in recovery as it should let you open your phones internal storage on PC but im not too sure if stock recovery allows it, (TWRP) does. Then copy whatever files you need. Let us know if you can get into fastboot mode aswell

It sounds similar to an issue I have just experienced with a friends 5T. In my case it's a hardware issue (seems like Wi-Fi has died). Just for kicks try to flash the Android 7 firmware. For some reason it allowed me to boot the phone fine and use it, albeit without any Wi-Fi. I sideloaded OOS 4.7.6 and that seems to be the last version 7. You could also use the older unbrick software. Academic I fear as I think you have a hardware issue.