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Boot loop after being dropped, now stuck in DFU

My granddaughter's iPhone 7 is now giving me headaches. Other than a small crack in the screen that's been there for a couple of months and hasn't caused any problems, her phone was in pretty good shape.

Was being the operative word here, right up until she took it skateboarding - with a predicable outcome.

After removing the crushed screen protector and finding there wasn't actually any further damage to the screen other than the original crack in the corner, I proceeded to dig in to evaluate the issue. She brought it to me because every time she turns it on, it displays the Apple logo for a minute or so then goes black only to repeat the scenario ad infinitum.

So I opened up the phone and tried every combination and permutation of plugging and unplugging all of the connectors on the motherboard to no effect; nothing I tried made any difference.

Finally I plugged it into a computer and managed to get it into DFU mode. It told me the phone couldn't be updated and had to be restored, so I said fine, go ahead and restore it.

No luck there; it quit with error 4013. I did have a replacement screen that I was holding onto for the inevitable time when the screen took more damage and truly needed replacement, so I went ahead and plugged the replacement screen in and tried the update again with the same results.

Now it no longer boot loops, but comes up in DFU mode every time.

I'm forced to conclude there's probably damage to the logic board and would like some help in determining what my next step should be. I have a certain amount of equipment such as a multimeter, soldering iron, heat gun, etc., at home but I can always go into work and use the professional grade equipment there like microscopes and hot air stations if need be.

At this point I'm considering the possibility of attempting to reflow the audio IC as demonstrated in this guide that actually describes the situation pretty well.

How to fix an iPhone 7 stuck / hanging on a logo - iFixit Repair Guide

However, are there diagnostic steps that I should try first? Things like diode checks? If so, what should I check, and what would I be looking for?

I've worked with circuit boards for about 40 years now and the delicate nature of iPhone logic boards will be a first for me, but I have no problem diving in and giving it my best shot.

I'm tagging @flannelist as the closest to an expert in this area that I'm aware of, but would welcome anyone else who has expertise in this area that's willing to help out.

Update 5/6/2023

Tried connecting with 3uTools. When flashing it gets to 19% after the "Sending KernelCache" part, changes to "Restoring", then fails with "ERROR: Device failed to enter restore mode."

Interestingly, when I click on the "Exit Recovery Mode" button on the Device page, it actually does exit DFU and goes back to its reboot cycle. Afterwards I put it back in DFU mode and it stays there.

Answered! View the answer I have this problem too

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@dadibrokeit I see you. I'm just finishing a stint of nitpicking a bunch of wikis and my brain is done for the night. But I will give this a good read in the morning. Or when I my brain remembers at 2am and can't let it go.

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@flannelist Thanks Alisha, no rush - I slapped together an iPhone X with a bad proximity detector and a busted screen for my granddaughter so she's got a perfectly good phone to use for the foreseeable future.

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@dadibrokeit

Hi!

Just until @flannelist get back to you I have some more info...

you mentioned that the phone was dropped so it could be the audio ic as you mentioned but it could also Nand related as bad Nand or a fault on that area can give a boot loop from what iv seen...it might be worth checking if there any any components missing from after the drop...I usually do this by comparing a image from ifixits teardown to the board infront of me

iPhone 7 Teardown

step13 to 16

just thought I would pass this by just to help with the process of elimination :-)

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@hellomacos That is a good point. At the very least I should be able to check the components on the top side of the board; on the bottom they're covered by some sort of insulating sheet so I'll probably hold off on removing those unless I get to a point where there's nothing else to do.

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@dadibrokeit

Yeah I would definitely leave that as a last resort

Best not to apply heat and possibly make the issue worse

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Sounds like you’ve gotten this as close to a confirmed board issue as you can (I suspect you’re right).

Well. The thing about the iPhone 7 is it loves a good boot loop. Could definitely be audio IC, especially with the drop, but the unfortunate thing about the lines that are typically problematic with Audio IC is that the problem spots can be tricky to get at for testing. And reflow could help, but more than likely if that’s the issue, you’re gonna need a crash course in BGA work. I use the method Jesse Cruz outlines at repair.wiki for these (you’ll have to scroll to the Audio IC section). Based on your 19% fail on restore though, I might look into NAND issues.

NAND power line shorts are super common, especially after a drop. They don’t usually boot loop for that, they just won’t boot. But I would look anyways. I would carefully scrape away the silicone over some of the components around the NAND just probe around for shorts (especially in the corners, for some reason). It’s just a thing that happens on tons of iPhones, heavy drop cracks a cap and then NAND can’t get the right power.

Having a DC power supply, or a DT880 (those can be had at eBay or other spots too, just linking for point of reference) can help to an extent, but interpreting boot loop power signatures is a science, and an inexact one at that. It’s a lot of pattern recognition, which means exposure to a lot of patterns.

I wanted to get this posted before I forget so I probably left some stuff out, but I figured this is a start

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Ah, I think you may have called it. Going through the repair.wiki article, one line in particular stood out:

If 3uTools Easy Flash gives you a failure at 19% or 20% and 3u Tools iTunes Flash gives you Error 9, then the issue is NAND. You'll need to replace NAND. NAND is the memory/storage chip.

I haven't tried the 3uTools iTunes Flash yet, but I certainly can. If I get the Error 9 on it, that would certainly seem to confirm Jesse's conclusion that it's a NAND problem.

Although I'm willing to give the reflow of the audio chip a try, removing, reballing and reprogramming a NAND chip is too far out from my comfort zone. I may give the reflow a shot, but at that point I'm going to be inclined to keep an eye out for a motherboard/home button replacement, whether that means just buying the parts or picking up a damaged phone that I can salvage those parts from.

Anyway, thank you all for your help; at least I have a better idea of what I'm dealing with now.

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Curiously enough but not surprising now that I think about it, using the 3uTools iTunes Flash resulted not in Error 9, but instead gave me the same Error 4013 that I got when actually using iTunes itself for the programming.

Next step will be to go over the board with a fine tooth comb looking for any damaged or missing components. Skeptical that I'm going to find anything that obvious, but can't hurt to look.

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@dadibrokeit Error 9 can be a lot of things, but so can 4013. I will be curious what you find. I have been so busy lately I haven’t gotten to do much board work and I miss it.

And If it makes you feel any better. I don’t mess around with NAND programming either. I am confident at my troubleshooting skills, but I am still working on my soldering all the time. Or as much as I can anyway. I haven’t found a reason yet for me to need to desolder a NAND chip. But I’m sure I’ll get there eventually.

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@flannelist Thought of you when I saw this on The Chive, Alisha.

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<3 appreciate it greatly!

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Jerry Wheeler will be eternally grateful.
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