What was your toughest diagnosis??

First off, sorry if this is posted in the wrong section. I just thought since this seems to be a learning environment with some great and knowledgeable people we could share some of our toughest fixes. Whether it be mechanical, electrical, a vehicle that kept coming back for the same issue, a scan tool that led you astray or just simply overlooking something easy.
I think some people just getting into this go looking for information and get overwhelmed with the knowledge that people here have. I thought this would be something to show them that we all started somewhere and still get out butts kicked.
I would love to contribute to this but there’s really not much I can’t do.
Well that’s a lie. I get my butt kicked just trying to tie my shoes in the morning. That’s why I switched to slip on boots.
I moved more out of the automotive field and into industrial electrical and instrumentation. I still have many examples I can add of tough fixes. That will have to wait until I have access to an actual keyboard.

e36 bmw m3. Misfire cyl3. pulled plugs, noticed cyl3 was wet. instant though of bad headgasket (s52 are notorious for this). pull all plugs, crank it, geyser of liquid outta cyl 3. (covered holes, started saving for new head/head gasket combo)

later on started thinking the fluid evaporated REALLY quick and it was hot outside... coolant doesnt evaporate that fast...
bought lab scope, inj3 bad, stuck open (no pintle hump in trace)... 90$ injector and a few hundo in scope parts, all well and good until it ate a clutch...

Some of the hardest problems I've solved have come down to bad new parts, especially as a new tech learning to have confidence in the diagnostic method.

You go through all the steps you can think of and you're 90% sure X part will fix the car and it comes back a couple days later with the same problem.
Now you have to question your methods, your results, your tools when all the while that shiney new, customer supplied, Rock Auto part is laughing at you.

Some of the hardest problems I've solved have come down to bad new parts, especially as a new tech learning to have confidence in the diagnostic method.

You go through all the steps you can think of and you're 90% sure X part will fix the car and it comes back a couple days later with the same problem.
Now you have to question your methods, your results, your tools when all the while that shiney new, customer supplied, Rock Auto part is laughing at you.

One that springs to mind from many years ago was a on a little Vauxhall Opel push rod engine . The symptoms had go progressivey worse over a period of two weeks or so he had already had it to the dealer but no joy. It would start fine but after a couple of seconds would progressively run rougher and quit after a few seconds. This was long before cats were fitted it was a really simple single carb 4 cylinder. Eventually we traced it to an alloy inlet manifold with a porus casting The inlet manifold was heated internally by exhaust gas the exhaust was leak right back to the inlet 100% EGR long before EGR was fitted to any european car.. The exhaust passages gave no clues to their presence until. you unbolted the manifold.

Another one was a brand new Chrysler Imp that had no oil pressure, traced to a plastic production plug found the oil filter housing assembly and the cylinder block. Chrysler engineering rep reckoned to be industrial sabotage !

I dunno if this was my toughest? But it's certainly the toughest one I can remember off hand. :lol: I've mentioned this one to others in the weekly meetup.

'04 Corvette comes in with inoperative drivers side mirror, power window and power door lock. Both are controlled by the Drivers Door Module. Turns out, my shop has replaced this module three times in the past few years. :dry:

Powers, grounds, Class 2, it's all good. No outputs are shorted. So what's cooking the modules? I eventually find two reversed connectors in the drivers door harness. One connector is for the speakers, one is for the drivers mirror. Swapping them effectively loops the speaker to itself, and connects the DDM to itself. Driver goes to adjust the mirror, and POOF, there goes the magic smoke.

I'm thinking, slam dunk. One new DDM, please! Shiny new DDM from the dealer arrives a week later - same thing. I'm completely stunned. I went over this stupid thing pin by pin. Went over it again. Checked the outputs again. Phoned a friend. Went on iATN. It all adds up to a bad DDM. Management is now convinced I screwed the pooch and missed something.

Another new module is weeks out, so we find a used one on Ebay. It works! Perfectly! Window, mirrors, locks, it's all beautiful. Which means I lost flat rate time AND sleep over a bad brand new dealer DDM.

I have to agree, when you put a new part in you want to be able to fix it, at least cross it off the list if someone else put it in.

I remember a year or two ago looking at a Titan that I failed NYS inspection for the hazard lights not working. Customer got mad because they never worked and passed it last year. Great. If my memory is correct, it's a pull down input to the BCM. I can't remember exactly how I went about this, but I know that providing the ground turned the lights on and I www confident that a new switch would fix it. It didn't. I think it was a four pin connector and one of the pins was in the wrong cavity. This was one where the problem HAD to be there and I'm pretty sure there was even an issue with the wire diagram not matching what I was working with, even when moving up and down a few years, and only figured it out by ohm testing the pins while pressing the button. Even after finding that the pins were wrong, none of them even went to ground, so I had to make my own. Very weird.



Nissan Tech

this was one of my early diagnosis's. Chevrolet Equinox, 2012 i think, customer said the motor liftgate didn't work. I didn't know that system very well but I made the mistake of trusting the customer and started digging into it. 3 hours later the panels in the back are all on the floor, I'm 99% sure i have a bad control module. but there is one wire on the module i'm just not 100% sure what its for based on how the diagram is written. now before i started checking wiring i got in the vehicle and looked for the button that opens the rear hatch, but i couldn't find it. so i thought, "maybe this model of vehicle doesn't have that?" so I've done my checks, i'm ready to call a bad module, but i can't get the one wire out of my head, and i determine that there has to be a switch for the liftgate in the car. i grab my light sit in the car, closed the door with the thought, "i'm not getting out of this seat until i find that switch!" 3 minutes later after carefully scanning the entire dash and center console and drive door, i found it. on the overhead console. I start laughing... toggle the switch, and the liftgate works perfectly. yeah, it would help if the customer turned it on.
lesson learned.

Not the toughest diag from a technical standpoint, but from an 'I know what the issue is but have no clue how to practically pinpoint it' one. It was a no crank on a mid 2000's Lexus LX 470, with the engine that had fully separate high and low pressure fuel systems with high and low pressure fuel injectors to match. It had the classic symptoms of a shorted sensor dragging down the 5v ref circuit and keeping the PCM from doing its thing. Alright, simple enough yeah? Eliminate sensors until I get my 5v back. If I only knew what was in store I would have told them to flatbed that turd right out of the lot. All of the information I needed from service manuals was lacking to start, all of the wire colors had been almost completely faded to nothing and the insulation on everything was super brittle. Many many frustrating hours of poke-n-hope diagnostics later and trying not to rip the intake manifold out to the tune of 5 labor hours to check the 2 sensors that live underneath it, I got lucky and found a junction block stuffed in a fuse box that sends the 5v signal out to multiple sensors via a connector with a shorting bar. Already fed up with this car I just cut the wires one by one until I eventually found the suspect sensor, and the automotive gods being the cruel things they are it ended up being the high pressure fuel rail sensor. 6 hours and quite a few choice words for the engineers that came up with that mess later and all was well.

I am on the toughest car right now it is an Opel Astra 1.6 L P300 code with freeze-frame for number 4 cylinder sometimes it will flag cyl 4 I have still not got the car to pass inspection because the money light is on. Fuel trims at idle great also full throttle is great but steady throttle total trim between 20-40 % running lean
Fuel pressure also 51 psi it is ok. I have a thread on the Opel in repair forum.

I am, of course, not a pro, but I can be cheap and determined.

I made it almost half a century without a power steering issue. I never had more than a loose or worn belt and I never learned much about PS. I had swapped cars with one of my sons so I was in a 2002 Subaru WRX. Suddenly the steering got extremely hard and smoke came out from under the hood. I had absolutely no doubt that it was the power steering pump. The hard steering came and went seemingly at random. It never smoked again, though I did put on a new belt and tension it. The factory service manual said to check the hydraulic PSI with a gauge. Yeah, that one got me laughed out of a few shops. Nobody had such a gauge or felt there was a need. A new PS pump had no effect. Was the new pump bad? The manual had me thinking it was the rack, so a grand plus a lot of work. I was having a tire on another car repaired and the tech tolerated my questions. He told me that I was on the wrong track and that in all his years he'd never seen a bad rack that didn't leak. So what's left? I scoured the forums and found someone who had the u-joint on the steering assembly go bad. Easy enough, I pulled it. It was binding, really binding hard, but only sometimes. I replaced it and voila, no more problem. The smoke? To this day I don't know but I'm thinking that the u-joint binding caused the PS pump to lock up and the belt was slipping. And the parts changer in me was ready to buy a new rack!

grrr