Error Code 65535: High Idle After Startup

Hello everyone,
I'm having the following problem with a 1997 (B5) Passat diesel, 1.9 TDI:
Sometimes, the idle speed is increased after starting.
The error does not always occur.
According to the VAG-Com diagnostic tool, the following errors are stored:
17978 Engine control unit locked, P1570-35-10, Intermittent (Reading coil or key?),
00522 Coolant temperature sensor (G62) 30-10-Intermittent interruption / short circuit to positive.
Could this be the cause of the excessively high idle speed (the engine control unit thinks it's terribly cold and increases the RPM)?
65535 Control unit defective 37-10-defective-intermittent Undervoltage caused by a faulty power relay? Should this be the cause of the error?
If so, where is the relay located, and what is its socket number?
Thank you very much for your help!!
P.S.: I hope I haven't made any grammatical errors - please excuse me, I'm new here .

Hello Gerd,

Yes, the problem with the coolant temperature sensor is very likely the cause of the high idle speed.

'Engine control unit locked': Check the reading coil, check the CONNECTION of the reading coil, and check the key.

"Control unit defective": This could be due to faulty control unit software or a defect within the control unit itself. Simply wait and see if the car drives normally once the other two errors have been resolved.

Best regards, Rainer.

I will check the sensor using the actuator test to see if the values are reasonable.
All the errors could be deleted.
Therefore, I believe the problem is likely caused by a faulty control unit, possibly due to a cold solder joint on the connector of the control unit or the relay. Where is the engine control unit located?

Thank you for the quick response!
Best regards, Gerd.

The actuator diagnostics cannot be used to test the sensor itself. The name "sensor" indicates that it is a component that transmits values, so it is not an actuator that can be controlled.

Regarding topic 17978, if there are no errors stored in the immobilizer control unit (see instrument cluster), then the immobilizer system is usually not directly involved, and the reading coil is therefore likely not the cause. If there are no other errors stored anywhere, you are more likely to find success in the area of the power supply to the engine control unit rather than in the immobilizer system itself.