Brake switch cause limp mode?
 

So I picked this A8 up yesterday, it drove back and for some fortunate reason it didn’t hit limp mode for about 90 Miles, kickdown worked if a little lathargic, lock up was fine and didn’t seem to slip at all.

Now the fault codes are the g195 implausible signal 17105 and brake switch electrical malfunction 17087 these codes where cleared and return by the way.

The reason I wondered if it’s the brake switch as if I cycle ignition and drive straight off the box stays out of limp mode, but if I hold the brake pedal for any amount of time at standstill the box usually clunks into limp mode, I can brake whilst moving without issue just when it comes to a stop, I’ve tried google and on here but not a lot of info, anyone else came across anything similar? Just the fact the box behaves like normal whilst running is what is throwing me regarding the speed out put signal issue.

Just incase, the car is a PFL 4.2 4 speed +++

Thanks guys.

That sounds like 2 different problems. The G195 issue would more likely give limp mode.

And on a PF (non-DBW) D2 you get a brake switch fault anyway unless you dab the brakes before doing a scan. So that may be 100% spurious.
See: http://wiki.ross-tech.com/wiki/index...7/P0703/001795

Would it only cause limp mode when stationary? Pretty sure brake fault is present as cruise doesn’t work iirc, but I will double check +++

Turn on ignition and start car, check TCM codes, press brake firmly and check again.
Then go for a short drive and check again with engine still running.
That should help clarify things. ;)

Cruise not working could be several things including a faulty stalk and faulty control module (it's a separate unit in the ECU box under the bonnet).

Remember the brake pedal switch actually has 2 circuits in it (definitely FL, presumably PF).

One circuit really only triggers the brake lights, the other informs the ECUs and they are ever so slightly set apart so that one changes just before the other as you apply the brakes.

You can check the operation with the relevant measuring blocks using VCDS, just can't recall which ones they are. (Used that to diagnose that mine was mis-adjusted and causing odd gearbox behaviour)
Edit: Engine measuring block 66, at least on FL. This is what I said nearly a year ago in another thread:

Thanks mate I’ll give it a go, PFL has the same type of brake switch with 4 pins.

My D3 exhibited similar symptoms a couple of times in London and I replaced the brake switch as I had read similar stuff but do you ever get poor starting when warm on the car ie refuses to turn over? If yes as well then it could very well be your Crankshaft Position Sensor on the way out which is what failed on mine eventually and cured that issue at the same time