[Nexus 5X Q&A, Help & Troubleshooting] Speed1908:
Probable Solution

I would like to share my experience on how I saved my Nexus 6P which was bootlooping with bootloader locked and inaccessible recovery mode.

The problem started when I had updated my phone with latest OTA update 8.1.0 (Oreo). While I was browsing fb after the update, suddenly the phone froze and restarted. And ever since that moment it begun to bootloop. I tried to access recovery from Fastboot so that I can clear the cache which would most of the time resolve the bootloop issue however this time around I was unable to go to the recovery mode too. I wouldn't been able to flash from fastboot since bootloader was locked. There is no way to get around this unless I would be able to load the OS and go into Developer option to turn on "Enable OEM unlock" in the settings.

Since I would not be able to flash the stock ROM or get into recovery mode, I contacted Google support for help. They directed me to Huawei Support. Huawei told that since my phone is out of 1 year warranty they would not be able to service this phone and would cost me nearly $400 for getting me a replacement. Spending $400 for 20 months old phone was absurd.

I started to explore online what are the options available given the tight situation that I was into. I came across few forums discussing about using a heat gun above the finger print sensor to throttle the CPU which may stop the phone from bootlooping. I used a hair dryer to heat the phone while the phone is continuously bootlooping. I would heat for about 7 - 10 mins to an extent where I would not be able to hold the phone anymore. After nearly 7 to 8 failed attempts spread over 2 weeks, the magic worked.

The last attempt was slightly different. Here is what I did -

1. Battery was fully drained

2. I started to charge the phone while I kept it outside my window for about 20 mins. Temp outside was around 15 degrees Fahrenheit. (Did this step since I read somewhere that keeping the phone in refrigerator and restarting would resolve bootloop issue) However it continued to bootloop.

3. While the phone continued to bootloop, I allowed the phone to charge overnight.

4. Next day I saw that the phone did not restart like how it used to. It was stuck at GOOGLE and would not restart.

5. I disconnected from the charger. Went to fastboot and switched off the phone.

6. I started to heat the phone using hair dryer with the phone switched off for about 10 mins. I held it was a cloth since it gets hot pretty fast.

7. Paused for a sec, turned on the phone (it was bootlooping) and continued to heat the phone for another 5 mins. (by then I gave up)

8. Stopped heating. I left the phone and went out to work on something else. After half an hour, when i returned I saw that the phone was completely switched off and was not bootlooping. I got excited since I know that the phone was fully charged overnight and there is no way that the phone would have died due to battery drain.

9. I immediately turned on by holding on to the VOL down button. I selected recovery from fastboot. To my surprise for the first time in 2 weeks I was able to go into recovery mode. I cleared cache and restarted the phone in Normal mode.

10. Phone booted properly and loaded the OS.

11. First thing I did was to go into developer options and turn on "ENABLE OEM UNLOCK" option. (By now I knew how important it was to have this enabled. THIS COULD SAVE YOUR $549 PHONE!!)

12. Took a backup of all the files and unlocked the bootloader.

I downgraded the OS to 7.1.2 (Aug release) which I felt was a stable one for Nexus 6P. I also did a h/w check and all the tests passed CPU / Memory is fine. I am still not sure what could have caused the bootloop but the fact that it happened after the Oreo OTA would make me to stay away from upgrading the Android version beyond 7.1.2.

Thanks,

Prasad