17704 - Error in Mapped Cooling System

It’s not a month out of warranty and the TT has had it’s first ever “check engine” light incident. Using a handy tool I ordered, I was able to discover the fault for about the cost of having the dealership diagnose the problem. The message is a little cryptic, but provides a good hint as to what is awry.

17704 - Error in Mapped Cooling System (usually temp Sensor or Thermostat)
P1296 - 35-10 - - - Intermittent

Googling the error code provides a plethora of results discussing what causes this error. The first step in confirming the problem was to figure out if the coolant temperature sensor was actually misbehaving. I cleared the code out and then had Eileen keep an eye on the laptop’s graph for the coolant temperature sensor as we drove to some errands.

Sure enough, as the engine coolant slowly rose to operating temperature, for a couple seconds at a time the sensor would send an absurdly low reading and then return to showing the correct value. While the problem didn’t look like it was happening that often and may not be enough to trigger another fault, it’s obvious the sensor is not feeling quite right (that is unless there are chunks of ice flowing around in the coolant system).

On the way back home we stopped by Chaplin’s VW and visited their parts desk. I had the part numbers, but the parts guy knew exactly what I was after when he read the description I had on the Post-It next to the part numbers. Apparently this isn’t too uncommon a fault for the 1.8T motor that the TT shares with some A4s and a variety of VW models.

Evidently the accounting for warranty repairs between Volkswagen AG and the dealerships encourages them to make the parts for frequently needed warranty repairs cheaper. In this particular case, the parts were $9.10. There was some suggestion that the mass airflow sensor was cheaper nowadays ($60 rather than $300) for similar reasons.

It turns out there are some nice articles online at AudiWorld about replacing the flakey coolant temperature sensor that causes this. I think I may try to document the repair process myself with some more pictures. We’ll see how that goes.