[Nexus 5 Q&A, Help & Troubleshooting] newbie856:
Hello everyone.

I recently attempted to install a custom kernel (ElementalX) on my Nexus 5 D820. During this process, I encountered a bootloop while installing TWRP, which is probably the event that has caused the nightmare my phone has been in the past few weeks. Essentially, ever since then, LTE has not been working on my phone, and the rest of the cellular connectivity has been quite bad. Actually, LTE does work, but only for about 10 minutes. After this, if it is turned on, it will crash ALL cellular connectivity, including phone calls and texts, until I turn it off.

Initially, I believed that the custom kernel was the cause. I therefore flashed stock ROM back, thinking it would solve the problem (spoiler alert: it didn't). I then tried flashing many different stock ROMs, from KitKat to Lollipop and pretty much any variety of Marshmallow offered. This still didn't fix the problem. After this, I tried flashing individual radios from http://xdaforums.com/google-nexus-5/development/modem-nexus-5-flashable-modems-efs-t2514095, which still did not fix the problem.

I then realized that the problem wasn't in any of the files normally affected by flashing a new ROM and started reading up on the EFS partition. From what I can gather so far on my phone, I believe that I have a slightly corrupted EFS partition and that this is causing the stability issues on my phone. I tried restoring from a good qcn file as suggested here http://xdaforums.com/google-nexus-5/help/imei-0-efs-backup-t2769023/post60421668#post60421668 and that has still not fixed my problem. I tried about 4 or 5 different qcn files, restoring in different order, wiping the block partitions like suggested here http://xdaforums.com/lg-g3/general/request-3g-2g-lte-d850-t3300908 and that still didn't work.

But I didn't give up just there! I read from this post http://xdaforums.com/galaxy-s5/general/how-to-add-rf-lte-frequency-bands-to-t2886059 that you could edit NV items individually and figured that maybe I could try to edit the LTE NV items to their default values. Turns out, they were at their default values and they are fine.

At this point, I thought I had tried everything, but while attempting an NV item backup using Qualcomm NV Tools in EFS professional, I noticed an odd error that kept happening. All of the NV items would back up properly, BUT, the backup process would crash for NV items 33799 to 33802 and 33810 to 33812, every time, no matter what. Also, exactly when I tried reading these NV items, the phone cellular network goes down for about 10 seconds, as it normally does when LTE is activated. I therefore believe that these NV items may be corrupt and that cellular network stops working whenever they are read, which for some reason only occurs when LTE is turned on (I'm far from an expert in this field, correct me if what I'm saying makes no sense).

At this point, I've pretty much given up on fixing the phone, but thought I would reach out to xda-devs to see if anyone had any experience with this. Not having LTE is not a huge deal, but I would be quite happy if I could bring it back.

What I'm asking: If anybody with a Nexus 5 D820 could try doing an NV Backup using Qualcomm NV Tools and see if NV items 33799 to 33802 and 33810 to 33812 crash their phone, that would be much appreciated. If I know at least that this is a problem unique to my phone, it can point me in the right direction. On the other hand, if it's widespread across Nexus 5 devices, my problem may be something else completely.

What I'd really, really, really appreciate: If a kind soul could give me a full EFS backup of their working phone, I would be extremely grateful. I'm pretty sure that this would fix all my problems, and I would be forever grateful to the kind person that does this. Of course, you may wipe your ESN/MEID/IMEI before sending the file.

tl;dr

LTE doesn't work since flashing kernel. Probably corrupt EFS partition. If some can check if NV items 33799 to 33802 and 33810 to 33812 can be backed up on Qualcomm NV Tools, that would be great. If someone can send me a full EFS backup of their Nexus 5, that would be amazing.