Asking this on behalf of a friend who has a Mk II Octy 2.0 TDI

When he drives to work on a cold morning, the temp guage goes up to about the 1/2 way point to show normal operating temperature - about 90 IIRC. This may take up to 6 miles or so depending on the weather, and I know this is not unusual on a diesel.

However, he has noticed that on occasions it then drops back down to the 1/4 position, perhaps over 1/2 - 1 mile. It will then take 2 - 3 miles to go back up to normal.

He feels the drop back to the 1/4 position tends to happen when he has a clear A road run travelling at 40-50mph. He doesn't see it if he is stuck in slow moving traffic.

I've read posts about faulty temperature senders, but apparently the dealer has scanned in and found no fault codes. He is going back in with it (car just out of warranty but problem notified while it was covered) but he just wondered if anyone else has seen a similar effect.

Is there a plausible explanation, based on the driving conditions described. If it was a faulty sender I would expect the problem to perhaps be more random.

Sounds like a sticky thermostat to me. When sitting in traffic you rely on the cooling fan to regulate the temperature but on the move you are forcing cold air through the radiator, if the stat is sticking you are not cutting off the flow of hot water to the radiator so the temperature will drop off.

As the stat is purely mechanical it will not register an obvious fault code.