Kerneels

Hi,

I have a 2008 Q7 4.2 TDI S-Line.

A few months ago it displayed an error: Turbo Control Unit faulty. The check engine light came on, the glow plug indicator flashed, and it went into limp mode. I took it to the dealer, who plugged in their diagnostic tool, and they said it's the left side turbo. But, after clearing the error it was 100% for about four months.

Then, it happened again. I took it back again, but this time the error would not clear. The dealer's advice was to completely replace the left side turbo at huge expense.

I figured it's worth a try to repair it first, so a local mechanic swapped out the control unit, which they had imported from Netherlands. They had it programmed and tested at the dealer, so it should be OK. However, the same error keeps being displayed.

They tried everything they could think of, including cleaning all wire connectors and having the turbo pressure tested. They even took the new control unit out and replaced it with a second one. No luck. For some reason the error remains.

It is now back at the dealer who wants to start replacing parts without knowing what the core problem is. I am afraid costs might run away with such a trail-and-error approach, and seeing that it is an old car it wouldn't make much sense to spend more than the car is worth.

Please help! I am out of ideas.

dreadlocks

How many miles? Are you consuming any oil?

I think you have VNT (Variable) Turbos which can have the variable actuators fail.

Kerneels

The car has 160,000km (around 100k miles) on it.

It doesn't consume any oil. I do have to top up the coolant at infrequent intervals. Not a lot though, so probably a small leak somewhere, and not sure if it is relevant?

Yes, it has the variable vain turbos. To get to them we had to pull the whole engine out, so while they were reachable we re-conned both and had them pressure tested. Both tested fine on the bench. This was when we first replaced the control unit on the left-hand one.

dreadlocks

Is this the code your throwing? 19804/P3348/013128 - Ross-Tech Wiki

Turbo failing at that low of miles is not typical, would expect 2x that much use before you need new turbos.. Sounds like a wiring fault if you already tried the module and tested the turbos, mebe that note for the 3.0L TDI is applicable to your 4.2

Kerneels

Thanks for the suggestion. We'll check the fault code on the car and advise the dealer to focus on wiring before replacing any parts.

If not wiring, do you think the ECU could be faulty? Or would a faulty ECU cause more elaborate messages?

dreadlocks

I'd think a bad ECU would be throwing a ton of codes about implausible signals or something.

since it was cleared the first time, but now wont go away.. smells electrical, probably hit a bump or something and some loose wire/plug finally broke.

Kerneels

Where can I find wiring diagrams for this car? I bought the full workshop manual with 8600+pages of technical details for $40, but it only has the diagrams for the 3.0 TDI, not the 4.2.

Roberto Muriglio

How did you solve your problem. I've the same and Noone can solve it.
Thanks for your time

Kerneels

I never found the diagrams, sorry.
It turns out the faults wasn't electronic after-all, but a leaky water pipe. Had it fixed and sold the car about a year later.

mikevictor

Hey sorry for digging this up but I just found this thread and I have the same fault code and it popped up right after a coolant line (the one that passes through the T-pipe and goes on to the coolant tank) got leaky on the highway.
I fixed my leak and bled the coolant system already but the fault code still remains and I cannot delete it.

Did your fault code just go away after some time?

I checked all the wiring and all the fuses I found plausible for the actuator (though I don‘t know which fuse specifically it is for the turbo actuator).

Thanks for any help I just can‘t get this fault code to go