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Car: 1999 A4 2.5 TDi Quattro

My coolant temperature gauge sits below 90. I replaced the blue 4-pin sensor with a genuine Audi part but no change, therefore I suspected the thermostat. Before I replace the timing belt, etc I thought I'd check the coolant temperature using the additional functions available through the climate control display (code 51c). This shows the coolant temperature climb steadily to 90 degrees and then it stays there, doesn't even drop to 89 degrees. This leads me to suspect that either my thermostat is incredibly accurate (in which case the gauge or wiring to it is faulty), or this function doesn't display actual coolant temp. Can anyone confirm the reliability of this method?

Correct, it doesn't show the true value of the coolant.

There is another measuring block using that method which I think shows the true coolant temp. IIRC one measurement is sent to the cluster and the other is sent to the ECU.

and remember it could be a gauge fault...

Problem solved - was the thermostat. Almost cost me an engine though, the new (genuine Audi) belt tensioner gave up the ghost after about 100 miles causing the timing belt to go slack, resulting in some scary noises and really uneven tickover.

The code 51c in the climate control is useless - it just climbs slowly to 90 regardless of coolant temperature, regardless of whether the engine is even running. It seems to control the fan/heater delay you experience when you start the engine from cold, which evidently doesn't wait for the engine to warm up but just waits for a predetermined time before the fan speeds up.