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Gearbox has started to ping up the solid red gear indicator in the cluster , just plugged in and retrieved this code

18155 - transmission solenoid power relay
P1747 35-00 open or short circuit to ground

No engine codes or abs codes

Cant seem to find much info regarding whether it's in the mechatronic or in a relay in the car somewhere so I can start checking power/ground etc.

Wondering if anyone has had this before?

If that power relay has any input via the F125 switch and that hasn't been replaced in a few years I'd probably start with that.

There's a fusebox under the bonnet for gubbins as well.

Couple of wiring diagrams in the Bentley manual that might help. Looks like there are a lot of earth points to check.

Is the fault always present or does it occur randomly? If you reset it, does it go away for a while?

This fault code is to do with the power supply to the solenoids so I'd check that the TCU is getting the right voltage first since they get their power from there. Measuring block 003 will tell you the supply voltage. If that looks OK, I would check the 16pin connector to the valve body for corrosion / damage.

If that all looks OK then I'd swap out the TCU to see if it's that. Not easy if you don't have a spare.

My feeling is that it's not the F125 but it's such an insidious little <beep> that you can never be sure.

Thanks for the replys gents , it happens all the time now , it was sporadic the first few days , will get the carpet up and check the TCU first then move to checking wiring.WIll report back if I find anything.

Also battery and alternator. Any power issues and they're worth looking at to rule out.

Wouldn't be surprised if you find water ingress to the TCU under the carpet. My money is on that or corrosion on the 16 pin connector.

Audi and their positioning of delicate electronics in the dumbest of places...

As if by magic ,passenger well ,bone dry ,drivers side foot well ,soaking wet ,water poured out of plastic box that TCM sits in , think I was just in time ,I can see where a power cable has been arching to enclosure which I think was throwing the short to ground code.

Inside of TCM is dry .just some green corrosion on connector pins but internals look fine
Will clean everything ,dry out foot well ,refit and test and will report back

What do I win?

If it were me, I would wash the lot with isopropanol to get the water out and let it dry thoroughly. And clean those contacts as well as possible with a tooth brush and isopropanol. Same for the mating connector.

Obvs the leak needs to be sorted.

I'm seeing a lot of corrosion on the PCB. You *definitely * want to clean that off and hope that it hasn't weakened the solder joints.

I'd be inclined to get it professionally cleaned and checked. You don't want it doing weird stuff with the gearbox.

Yeah, I'd be feeling glad that I had a spare TCU at this point.