Driving in my 1.9TDi A4 yesterday through lots of rain, I got off the M1 and the esp light came on, and the glow-plug light started flashing, leaving me in limp mode (1100rpm and no throttle) and a max speed of ~40mph.
Whilst waiting for the AA man I sat with the heaters on my feet, and the problem stopped...
So, here's a quick guide to save you from going through what I did!
ESP Light on, Glow-Plug Light Flashing, Loss of power, In limp mode, 1100rpm- usually happens in wet weather, or just after washing the car. Carpets being damp in the footwell are an early warning sign- heed the warning!
In some circumstances you may find the rear brake lights don't work (mine still did).
Drainage holes blocked, or ECU housing seal refitted incorrectly. Eventually this allows water into the cabin and meets the 4 vehicle interconnects, OBD, brake/throttle pot.
Once the circuit shorts on the brake/throttle pedals, the EMS gets the hump, there's a rationality check done between the rising and falling potentiometers on both pedals, if these don't correspond, the EMS will put the engine into limp home mode.
The AA diagnostics didn't show anything- I believe VAG-COM/Diagra etc will show a brake pedal switch fault.
If this happens to you on the move- put the heated footwell on full and wait for an hour or so. Beware the reason this is happeneing is water has shorted electronics, removing the ESP fuse won't get you out of limp-home mode either, so proceed with caution.
General Toolkit (Sockets: 10mm,16mm) Various Spanners, 6mm Allen key
Petroleum Jelly (Or Grease)
1) Remove the scuttle tray cover and remove the battery (10mm Spanner & 6mm Allen Key)
2) If not already done, remove both drainage holes and clean/enlarge or remove completely:
Under the battery:
Under the brake servo:
3) Clean around both holes and remove and leaf matter from the scuttle area. Follow the next steps if you still have a problem with water in the cabin.
4) Remove the driver's side wiper (16mm Socket):
5) Then use a T30 Torx bit on the 5 securing screws- mine only had 4 on, it was obviously too much of a pain for Audi to do the job properly in the first place...
6) with the top half of the ECU housing removed inspect the seal:
It's probably past it's best, I'm waiting until the New Year and I'll order one from Audi- will post up Part # & Cost when I do.
7) Unclip the ECU and remove the right hand connector- this should be placed ontop of the engine out of harms way.
8) The lower half of the ECU housing is where the problem lies, there's a rubber seal between the body and the plastic housing, if this has perished, or been refitted incorrectly, it will let water in. Remove the housing using a 10mm socket and an 8mm one for the front nut.
9) Use a 10mm socket for the nut securing a positive feed (the nut is captive so it won't drop)
10) Unplug 4 interconnects; White, Black, Red and Beige (these can only be fitted in one direction so don't worry too much about remembering)
11) Lift the lower housing clean of the scuttle:
12) Left this- you can see the water/leaf mess which was causing problems:
13) Inspect the rubber seal, and replace if necessary, or clean and reuse. To avoid the seal dropping out whilst refitting I used a small bead of gasket sealant to hold it in place:
14) Clean the mating face on the body:
15) Refit the seal:
16) I then used some vasilene on the mating face for extra security- I didn't want this happeneing again:
17) Refit in the reverse of removal, making sure the seal is located properly, otherwise this is all in vain.
18) Refit battery and re-code stereo/drive to get the ESP light to go off.
19) Dry out footwell and inspect connections on pedals for corrosion/water damage.
All in this took about 2.5hrs, I left a fan heater drying the footwell carpet out whilst I was working. Hopefully this helps people with a common Audi problem, if you know your car has had a replacement wiper motor, or any work done in that area, I'd inspect the seal- it's never nice to be stuck with a car in the hardshoulder on December 23rd...