F125 Switch or TCU - or something else?

[Duke Thrust]

Hi all,

Recent RS6 purchase here, 2003 Daytona Grey - loving it so far.

But... Gearbox niggle has surfaced. It's done 130k and had a full gearbox rebuild at 90k. Gearbox oil and filters changed by the previous owner less than 1k ago.

A couple of times now just after starting from cold all the PRND gear indicator on the dash has lit up, a switch off and back on and the problem has immediately resolved itself (seems to be after the car has been standing for a few days on both occasions).

Some searching on this forum suggests it could be the F125 switch or the TCU itself, but wanted to get people's thoughts.

Thanks!

[Shoppinit]

Could be a number of things. Hard to say without seeing what the fault codes are.

Does it go into fault after starting or when you put it into gear?

[srichards]

When was the 12v battery last changed? A dodgy one of those can cause all sorts of mayhem as well.

Is it kept outside or in a garage?

[Duke Thrust]

Could be a number of things. Hard to say without seeing what the fault codes are.

Does it go into fault after starting or when you put it into gear?

It's not showing any codes at the moment, from the transmission at least (two engine mount codes, looks like they've failed)

It goes into fault after putting it into gear, both occasions it's been into reverse first.

[Duke Thrust]

When was the 12v battery last changed? A dodgy one of those can cause all sorts of mayhem as well.

Is it kept outside or in a garage?

The battery is pretty new, last owner changed it, I think in the last year or two.

It's kept outside.

[Mɐʇʇ]

On the surface of it, those are the symptoms I had when my F125 switch needed replacing, but as Shopp says, you need to verify/check the actual error code via VCDS.

[Shoppinit]

Yeah. There should be codes stored in the TCU if you've had the PRNS on the dash.

[Duke Thrust]

Thanks guys, I'll wait for it to happen again and see if it records a code that time- I'm just using a standard reader (that will talk to gearbox/ABS etc etc) not Vag Com (sorry if that's the wrong name), as I say, no codes so far.

[Shoppinit]

It's possible that your reader won't be able to read the manufacturer specific codes of the TCU. Where abouts are you? Maybe there's someone near you that can scan the car for you.

[Mɐʇʇ]

I'm 90% sure that a standard OBD reader won't read those TCU errors.

[Shoppinit]

Also, since the defect is effectively self clearing, then you'll have an intermittent or pending fault code which might not be displayed by your reader, even if it can talk to the TCU.

[Duke Thrust]

Ah, okay, makes sense.

I'm in Cheltenham.

On the F125 switch, reading this forum the part number (seems to be) 01L 919 821 B, when I google it the results come up as 01V 919 821 B

Is that correct?

[Mɐʇʇ]

Looks like there's still stock too, should work out about 180ish quid

[Duke Thrust]

Looks like there's still stock too, should work out about 180ish quid

Thanks - from Audi?

(Searching reveals two different connector types, one round and one oblong - both seem to have ten pins.

[Mɐʇʇ]

Yes.