Temperature Sensor Error Affects Engine Performance

Good evening.
I'm having a problem with my Fabia, 74kW TDI, manufactured in 2001, with approximately 40,000 km on the odometer. The temperature gauge is fluctuating from normal to extremely cold – recently, during a trip from Dortmund to Salzburg via Munich, covering about 500 km. Once, when the outside temperature was 4 degrees Celsius, it went into the red zone while the heating and ventilation were turned up full blast. I had the thermostat replaced since then, there's been no more red zone, but the gauge still repeatedly drops from normal to extremely cold and then returns to normal or slightly below within 5 seconds.
Recently, after driving 50km and refueling, the engine started in "limp mode" (1200 RPM idle). I've also repeatedly noticed that when the temperature gauge shows "very cold," there's a characteristic "surge" when releasing the accelerator – meaning the engine doesn't immediately decelerate, and the fuel consumption doesn't drop to zero instantly, but instead, diesel continues to be injected for about 1 second.

My mechanic says it could be anything from the control unit to the temperature sensor or even the instrument itself.

Does anyone have any advice before I waste another bunch of euros?

Thank you very much.

sturmi68 wrote:

... Does anyone have any advice before I waste another bunch of euros?

Replace the temperature sensor... it's the cheapest option and likely the cause of your problem in 90% of cases. The other 10% could be a cable issue.
If it were AI, your engine management system wouldn't react to an "overly cold" engine.

I have now encountered five cases where the dual sensor was defective, but always only one part of it.
The part to which the MSTG is attached.
If both are malfunctioning, I would start by checking the wiring.

Best regards,
Rudi

Rudi wrote:

In the meantime, I have had five cases where the dual sensor was defective.

Only five? ...

I have more notches on my wrench .

Rudi wrote:

I have now had five cases where the dual sensor was defective. However, it was always only the part to which the MSTG is attached. If both are malfunctioning, I would first focus on the wiring.

The 9N platform (also known as the Fabia) apparently no longer uses dual sensors; instead, the main control unit (MSG) distributes the water temperature data to all sensor modules (STG) via the CAN bus.

-> If the sensor in the AI system malfunctions, the probability is correspondingly higher (compared to systems with dual sensors) that the MSG will also calculate incorrect temperatures. In such cases, the typical (and well-observed!) symptoms associated with this type of malfunction would also be present.

Thank you for the tips. I went to Skoda, and the temperature sensor was showing all sorts of values between -86 and +70 degrees. The only comment they made was that they charge €65 plus 20% VAT for 5 minutes of work and a 15-minute test drive.
Thank you all very much.