cparke

After changing the engine coolant temperature sensor as a "shot in the dark" to try to fix too rich fuel trim problem, now the ECU is doing the strangest thing that I've never seen it do before. (coolant is not leaking from the sensor, and the sensor's temperature values seem correct)

Drive the car with some highway and the engine learns a too rich multiplicative trim of -24.2% and -25%, then turn off the engine. When you turn it back to ON (don't even have to start the car), it is back to 0% and has to re-learn! Once the MIL is on, I think it stops re-learning too and maybe will just stay stuck at 0% until the Load Calculation Cross Check fault code is cleared. Also, clear that fault code while the engine is running and it immediately reappears on the same screen right below the line where VAG-COM says the codes were cleared.

Could the ECU chip for managing fuel trim be going haywire? How hard is it to swap the ECU with another car to see if that is the real problem here?