Hello everyone I have a 2000 C5 A6 2.8L Quattro and I’ve been having a problem that’s affecting that ability for my car to be a daily driver it gets worse by the day. I really need anybody to point me in the right direction or if anyone has had a similar or the same issue I would love to hear what they did.


Thank you, appreciate it.
My problem:

There is an electrical Issue/failure with the car. Intermittently, when key is turned to on/acc mode it goes into safe mode/limp mode and a click can be heard from the engine bay. No EPC light, no check engine light. In this state you can crank the car until the battery dies.

fuse 28 fuel pump, fuse 29 engine control, fuse 34 engine control do not get power on acc mode. fuse 36 fog lights no power.

Symptoms are all PRND234 are highlighted as it’s in safe/limp mode you can here one click or rather three small fast clicks from the engine bay. If it’s like this you can crank it until the battery dies. As fuse 28 29 & 34 get no power. Another symptom is without pressing the brake pedal you can move the gear shifter. (Brake light switch on gear shift goes off when it’s in this safe mode)


Also intermittently when the key is turned to the Acc mode/ on position, the EPC light and check engine come light on ( no longer in safe mode) fuse 28 29 & 34 get no power in on/acc mode as well as fuse 36 have no power. But it will start without hesitation, probably 2 cranks and starts on the 3rd like the first day I got it. No limp mode and the car will drive 100%. Shut it off and it will be in safe mode again with the exact same symptoms.

When vehicle is idling and started every fuse is getting power except fuse 36 fog lights (not worried about that)



Engine Management Code Scan results. 16989. P0605, Internal Control Module ROM error. P1603 Internal Control Module failed self test, intermittent.

Transmission code scan results. 18258. P1850 Powertrain Data Bus missing message from ECU. Intermittent .

Also had engine management code P0322 for the crankshaft position sensor. Got it changed and didn’t do anything but clear the code and it didn’t come back.

The main code that I’ve been having problems with is P1850 and occasionally P0605 will come up. But these are causing the no start issue. My mechanic hasn’t been of much help, he said it’s “possibly” the TCM and the ECM intermittently not communicating. But he didn’t even check my relays?



what I have inspected and done this far.

Checked all fuses in interior fuse box, fuse 28,29,34 do not get power on acc mode, but when started they all have power. Have yet to check voltage from fuse box.

Inspected transmission control module, pulled apart door trim and carpet to expose the TCM box, it was completely dry on the outside. gasket was 100% and the module looked brand new. Looked untouched on the inside of box no moisture in TCM case or on the module itself. Not even a fingerprint on it. All wire leading into it visually looked fine all pins on connecter and Module side were fine no corrosion, water or fried or bent pins.

Diagnosis on internal relay box, relay 4 fuel pump relay looked visually fried, removed and codes + safe mode disappeared when codes cleared they didn’t come back. No EPC light no check engine light. However after letting It sit for five minutes code came back. Along with limp/safe mode. replaced relay with slightly better one code cleared and came back after a few cycles of the ignition. Was excited when it cleared but came back right away.

At first when relay no 4 fuel pump 352 was taken out and looked visually fired, cleared code on OBD and it did not return until later that evening. Even with the new used relay, same result. Leading me to believe there is a problem with the connection to relay 4 or and improper ground. Both I have yet to test.

Code P1850 returned and vehicle back to safe mode, can cycle the ignition a few times and sometimes it will not be in safe mode ready to start but intermittently getting worse.



This is essentially making this car un-drive-able if you’ve read to this point I would like to say thank you. In stumped at this point with this car, but if anyone could point me in the right direction any info would be greatly appreciated. Thanks again hope to hear from you guys