Hello from Canada. Last year I purchased a 2016 TT S Line. Recently I have gotten a P2556 error code indicating coolant level sensor issue. Audi tech feels that the thermostat needs replacing, but the coolant level sensor is in the expansion tank. The temperature gauge is erratic going from 50 to 100 degrees but usually down near 50. Also just recently noticed that the cruise control does not work. I found an old thread from 2018 from another member who had identical issues and symptoms but don't know if the error code and cc issues are related.

Questions, if replacing the thermostat should I do the water pump as well? car has 176K km. Cost of thermostat unit is $800Cdn and water pump another $250 and quoted 5-6 hours labor at $185/hr.

Thinking of replacing the expansion tank as it appears that the coolant level sensor is not easily replaced into an existing tank. Cost of new tank is only$60 so not too much wasted if it doesn't work.

Any ideas or suggestions would be appreciated.

thanks

Randy

As for the thermostat - since the temperature gauge is behaving strangely (jumps between 50 and 100, but often ‘sits’ close to 50), it really looks like the thermostat is either not closing or is hanging in the open position, causing the engine not to reach the correct operating temperature. This can also affect the cruise control - in some Audi/VW models, when the engine doesn't reach the right temperature, the cruise control doesn't activate or is turned off preemptively.

I would do this : 

Replace the tank - a cheap attempt to eliminate the sensor.

Thermostat - rather necessary, symptoms very typical.

Water pump - definitely worthwhile by the way, preventively.

Cruise control - may be linked to engine underheating, worth waiting to see the effect after replacing the thermostat.

Thanks for the inout