No-Crank, No-Start Issue with Audi A6 2010 3.0T

[slow_a6]

Hi all,

I'm dealing with a no-crank, no-start situation on my Audi. Unfortunately, I don't have VAGCOM but am using OBD 11, which isn't ideal, but it's what I have for now.

The situation:
I made a mistake by leaving my windows down during a severe rainstorm. To make matters worse, I also left my keys in the ignition (accessory mode, engine off). By the time I realized this, the battery was completely drained. It took multiple attempts and three different vehicles to jump-start the car.

On the third attempt, when I tried starting the car, the dash went haywire with flashing error messages and lights, and the electronic parking brake (EPB) cycled on and off several times. On the fourth attempt, the car finally started, and I drove it home. However, I noticed significant power loss, especially under load, and the RPMs were limited, which I assume was "limp mode."

The next day:
The car wouldn’t start again, though everything else (lights, dash, MMI, etc.) seemed to be working. When I try to start the engine, there's just clicking under the steering column.

What I've done so far:

Checked fuse boxes (driver's side, passenger side, under the wiper plenum, and trunk) – everything looks good.
The alternator tested as weak but still functional.
Installed a new Bosch starter.
Replaced the battery with a new H8 AGM battery, which shows 13.9V on the multimeter.
Checked all grounds; they look good. However, the ground near the wiper fluid cap (under the plenum) is sitting in a small puddle of water and some gunk, though it appears intact.
I realize I could take the car to a shop, but I have other means of transportation and don’t currently have the budget for that. Since I’m mechanically inclined and have completed all the work mentioned above without issues, I'm prepared to keep working on this for as long as it takes.

IMG_5969.jpgIMG_5983.jpgIMG_5965.jpgIMG_5966.jpgIMG_5982.jpgIMG_5981.jpg

I'm getting frustrated but I know from past Audi ownership experiences that sometimes it’s just a silly module triggering the other codes. It is a beautiful example of an A6 with a lot of preventative service done, dealership maintained from new to 102K, that's when I received the car from my father and continued with extensive preventative services the car now sits at 175K. I understand that I will have to call it quits at some point.

When this first started, I scanned the car and got 50+ codes. After installing the new battery (which I coded correctly) and clearing the codes, I rescanned and now only have 4 codes left. As shown above.

Any advice or suggestions would be greatly appreciated.

Thanks in advance for your help!

[King_Tutt]

I would suggest pulling up the carpet on the driver and passenger side to make sure there isnt a module in water currently. Also if you have water sitting in the firewall area drain that too. How long have you dried out the vehicle? Are you also able to test with a multi-meter if the stater is getting voltage to the signal wire?

[Avantly]

Most of those codes are likely due to the battery dying, the steering angle sensor one certainly is and should go away with a short drive.

It appears the MMI unit is saying no communication, it is likely you have water pooled under the drivers side carpet which has gotten into the bluetooth module. Does the MMI work at all?

The air bag CAN bus communication is likely also caused by water, but I am not quite sure where the easiest place for water to affect the air bag module is; That module is located on the top of the tunnel, underneath the center console so it should not have been possible for water to reach it. Since the code is intermittent I would wonder if maybe it is caused by a grounding issue and the airbag ground is likely one of the grounds on the outer edge of the drivers or passengers footwell (I am not looking at schematics right now). Or maybe the power feed if water was able to enter one of the fuse or relay boxes/panels.

The ABS code 01316 seems to indicate communication issues as well; The Air bag module is on the same high speed CAN bus, therefore both of these communication codes are likely caused by the same issue - This could be one of the modules suffering from water ingress, knocking out the bus, or one of the modules suffering ground or power feed issues, causing a voltage offset on the bus. The ABS control module is part of the ABS pump under the hood and very robust, it is unlikely this module is the source of the communication issue so I think the problem is not related to the ABS module itself.

I think the energy management code from the CAN gateway is from the battery going dead, just like the steering angle sensor code. However, the steering angle sensor code may not go away if the ABS module is having trouble communicating.

I have a hard time believing that water could have reached the CAN gateway behind the glovebox, however, I am looking at the schematics and basically all your CAN bus signalling going through the "CAN Bus disconnection plugs" on the left and right of the car before going to the gateway. The air bag module connects through the right disconnect, and the ABS connects through the left disconnect. Were both windows open? These disconnect plugs are visible with the fuse panel covers on the left and right side of the dash removed (only visible with door open). The CAN disconnects are decent sized rectangular connectors with purple latches on them. I would be inclined to disconnect both of these and closely inspect for water ingress (and thoroughly dry if any is found) before moving further with diagnosis on those codes.

For the water which is almost certainly under the carpet - If you are creative and careful, you can remove the door sill trim without removing the B pillar trim; You will need to bend it to do so but it's usually quite flexible. For drivers side, to remove this trim you need to remove the hood latch handle which is held on by a clip that is on the back side of the handle, you simply pull the clip towards the front of the car until it pops out and the handle will come right off. For both sides, you need to remove the little ~10" piece of upper trim that sits at the front edge of each fuse panel cover door, remove the fuse door and then remove this trim by pulling it straight out, starting at the top. You can remove the drivers foot bolster bracket or whatever it's called without removing the under-dash cover you just have to sort of tilt it after sliding it upward and sneak it out of the back of the under-dash cover. With the sill trim removed, you can slide the seats all the way back and lift the corners of the carpet for inspection. If you find a lot of water I'd be inclined to remove the front seats, get a shop vac and some towels and start drying. If you stick a shop vac wand up against the foam under the carpet (that loves to soak up water) it is actually pretty good at sucking most of the water out of the foam pretty quickly, just make sure to give the shop vac a rest to breathe so that it doesn't overheat.

If your bluetooth module in the left side floor box is soaked and done for, replacements are plug and play, no coding necessary.