Car is a 2012 A6 3.0T. I bought it with a salvage title because someone had taken it joyriding offload and hit the TCU. A few years after I purchased I ended up having to replace the transmission. It was done by a Local independent (makes and models in Salt Lake City) Drove fine for another year. Then the other day I got in the car, hit the start button and heard a bunch of clicking coming from the far back left of the dash. Engine never tried to start and the transmission error "transmission malfunction: you can continue driving with limited functionality" came up on the display. The igniton was on, but the car was not. Pushing the power button again did not turn the ignition off or start the car. I had somewhere to go so I left the car on and took another car. When I got back the car was off and started right up with no problem. I drive it around a little stopped and started it with no problem. But the next day the same thing happened except this time the car would start for about 2 seconds then turn off and give me the same error. Wait an hour or so and it might start.
I took the battery to autozone and it tested fine so I took the car back to the shop that replaced the transmission. They found some low voltage errors so the replaced the battery. They were not able to re-create the problem, so I took the car home crossing my fingers that it was just the battery. Next day (today) I drive over to a neighbors stay there for about an hour and drive back with no problems. The car is due for an oil change so I go to move the car into my garage to change the oil and it starts then dies with the same error. I decide to change the oil anyway in the driveway with the ignition on. when I am done the ignition has turned off again and it starts right up no problem. Drive it around again and start up a few times with no issue.
The independent shop found codes on the transmission control unit (TCU) and were able to link them to a technical service bulletin (TSB) that stated there was a communication issue between the car and the key causing the immobilizer to activate. The fix is an update to the comfort module? that could only be done at the dealer for >$150 and possibly a new key ($500) so I had the update done and so far so good (1 day) hoping that fixes it.
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