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[GUIDE] Fix PERSIST.IMG Loss Of Finger Print Sensor

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By antnyhills, Senior Member on 1st July 2020, 07:37 PM
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New video guide below

VIDEO GUIDE


1. *VERY IMPORTANT* We need to back up your broken persist.img and store it in a safe place. PC preferred. We will need this later. Do not leave the backup on the device as the next step will wipe the device.

*IF YOU LOOSE THIS PERSIST.IMG BEFORE WE MODIFY IT WITH A CALIBRATION, YOU LOOSE ALL CHANCE OF FIXING THIS YOURSELF*

*IT HOLDS IMPORTANT FP CALIBRATION DATA THAT WE WILL BE OVERWRIGHTING IN THE NEXT FEW STEPS*
Note: This requires root
Type this into terminal or ADB shell with root privileges.
Code:
dd if=/dev/block/bootdevice/by-name/persist of=/sdcard/persist.img
Backup the partition to another device. Preferably a PC as you may need one in the later steps.

2. Next we have to restore our OS to an earlier OOS. (10.5.4 is what I used and recomended)
Use the MSMTool to restore your device to 10.5.4.
Note: You will loose your unlocked bootloader and will be required to unlock your bootloader once more.
Guide and Source for MSMTool
MSMTool 10.5.4 Download

3. Next step is to unlock the built in 'Factory Mode' I have a guide on how to do this below.

[GUIDE] Unlock Factory Mode *root*

4. Now we need to perform a FP calibration. We want to fail.
  • Enter Factory Mode as shown in the previous guide. (*#801# in the stock dialer)
  • Swipe over to "Device Debugging"
  • Select "Finger print test"
  • Select "Finger print calibration test(Pure White - 90% Blue)
  • Select "Start Test"
  • Keep on going through the tests (You must hold an object or your finger over sensor on all BUT THE BLACK RUBBER TEST of these test for it to work!)
  • The test will fail and this is what we want. There should be a lot of red lines, if you only have one or two try again.

5. We need to extract the the old broken persist.img
NOTE: It is important that we extract the backed up persist.img as it has been unmodified by our recent calibration. This persist.img although broken still has all our working calibrations on it including Finger Print calibrations.
The way I did this is by opening it extracting it via 7-ZIP on my PC
Once extracted move the 'DATA' folder and the 'ENGINEERMODE' folder to the root of your sdcard.

6. Download a root explorer. I highly recommend 'Solid Explorer' from the Play Store.
  • Open 'DATA' folder and delete 'gf_persist.so'
  • Select the 'DATA' and 'ENGINEERMODE' folders you just put on the device and place them at /mnt/vendor/persist/
  • It should ask you if you want to keep the old or new files. Select to keep the new.
  • Reboot.

7. If you followed all my steps correctly this you should be able to go into settings and add your fingerprints.


Why does this work?

Good question... I think what is happening is something in the persist partition breaks which causes the device to think that the sensor is not calibrated when in fact it still is.
When we start and fail a new calibration this fixes whatever was broken in our partition. But now we have a failed calibration.
So what we do is take the files from our backed up persist partition and move over all the necessary files that have our original calibration. But now our partition has the part of it that wasn't working fixed.

I spent a good many days working on this fix (1-2 hundred hours or so). I hope you can be respectful if you have issues. Please kindly ask below and I'll do my best to help.
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2nd July 2020, 03:16 AM |#2  
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Thanks worked on.my TMobile OnePlus.
2nd July 2020, 02:25 PM |#3  
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I can confirm working on OOS 1.5.6 too. I downgraded from 10.5.9 with downgrade rom, which restores OOS to 10.5.6, and followed your guide and it worked as a charm. The only problem is that i cannot lock back the bootloader, cause it breaks FP again. Did you maybe try to lock it to try? Nice work man.
2nd July 2020, 03:54 PM |#4  
T-Mobile OnePlus 8 here. Stock firmware, orginally had my bootloader unlocked, rooted with TWRP and Magisk, backed up modem, converted to international firmware (10.5.7), lost signal, had to restore modem, ota'd to 10.5.9, unlocked bootloader again, rooted, etc. All while having a working FP the whole time. Ended up flashing EdXposed via Magisk, broke SafetyNet, etc, did a factory reset on TWRP (3.4.0-1 test), wiped data, rebooted recovery, rebooted system, booted back to TWRP. Installed Magisk to see if it'll boot with root, same thing. MSM'd back to T-Mobile firmware, then converted to international again. After the conversion was done and I unlocked bootloader and rebooted to system, my FP was broken. Ended up MSM'ing back to T-Mobile stock to see if it would work again, nothing.

TL;DR does this method only work if I had a working FP to begin with like it did stock out of the box?
2nd July 2020, 04:02 PM |#5  
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Quote:
Originally Posted by TheKnux

T-Mobile OnePlus 8 here. Stock firmware, orginally had my bootloader unlocked, rooted with TWRP and Magisk, backed up modem, converted to international firmware (10.5.7), lost signal, had to restore modem, ota'd to 10.5.9, unlocked bootloader again, rooted, etc. All while having a working FP the whole time. Ended up flashing EdXposed via Magisk, broke SafetyNet, etc, did a factory reset on TWRP (3.4.0-1 test), wiped data, rebooted recovery, rebooted system, booted back to TWRP. Installed Magisk to see if it'll boot with root, same thing. MSM'd back to T-Mobile firmware, then converted to international again. After the conversion was done and I unlocked bootloader and rebooted to system, my FP was broken. Ended up MSM'ing back to T-Mobile stock to see if it would work again, nothing.

TL;DR does this method only work if I had a working FP to begin with like it did stock out of the box?

Just follow the guide, you'll have it working. You just have to backup your current persist
2nd July 2020, 04:03 PM |#6  
Quote:
Originally Posted by dzontra83

Just follow the guide, you'll have it working. You just have to backup your current persist

So back it up even though it's broken? I'm on T-Mobile stock again, locked BL. Should I unlock, root, then pull persist and back it up?
2nd July 2020, 04:46 PM |#7  
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Quote:
Originally Posted by TheKnux

So back it up even though it's broken? I'm on T-Mobile stock again, locked BL. Should I unlock, root, then pull persist and back it up?

Exactly, a T-Mobile user confirmed guide to work, check 2nd post
3rd July 2020, 04:15 PM |#8  
While on 10.5.7, unlocked bootloader, I flash TWRP, then reboot, and it boots back to TWRP. When I flash Magisk 20.4 zip in TWRP, it goes to boot, then enters CrashDump Mode. When I try and use the flash-all to downgrade to 10.5.4, I get a bunch of fastboot "cannot flash critical partitions" errors, it fails, and goes back to CrashDump Mode. What am I doing wrong? Also, is it really necessary to downgrade to an earlier version of OOS to use Engineering Mode or can I use 10.5.7?
3rd July 2020, 05:09 PM |#9  
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Quote:
Originally Posted by TheKnux

While on 10.5.7, unlocked bootloader, I flash TWRP, then reboot, and it boots back to TWRP. When I flash Magisk 20.4 zip in TWRP, it goes to boot, then enters CrashDump Mode. When I try and use the flash-all to downgrade to 10.5.4, I get a bunch of fastboot "cannot flash critical partitions" errors, it fails, and goes back to CrashDump Mode. What am I doing wrong? Also, is it really necessary to downgrade to an earlier version of OOS to use Engineering Mode or can I use 10.5.7?

When you unlock bootloader, only boot into TWRP, then install magisk, i thin it Is still recommend to only boot into TWRP, not to flash it.
3rd July 2020, 05:20 PM |#10  
Quote:
Originally Posted by dzontra83

When you unlock bootloader, only boot into TWRP, then install magisk, i thin it Is still recommend to only boot into TWRP, not to flash it.

Thank you so much, that worked! New problem, what do I do since I can't downgrade to an earlier OOS (10.5.4)?

Performing the test on T-Mobile 10.5.7, when performing the test, nothing is failing after pressing start test. Its all passing, what should I do?

When trying to downgrade using the 10.5.4 MSM Tool. it gives an error of "Unsupported Target TMO" and "param preload" and fails.
3rd July 2020, 06:30 PM |#11  
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Quote:
Originally Posted by TheKnux

Thank you so much, that worked! New problem, what do I do since I can't downgrade to an earlier OOS (10.5.4)?

Performing the test on T-Mobile 10.5.7, when performing the test, nothing is failing after pressing start test. Its all passing, what should I do?

When trying to downgrade using the 10.5.4 MSM Tool. it gives an error of "Unsupported Target TMO" and "param preload" and fails.

Did you follow the instructions in test exactly? In second test you must't press the sensor.
Can help you with TMobile model, I have international one.
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