How big an input does the outside air temperature have on the ECU?
I'll tell you why I ask.
Before changing my timing belt, the temp gauge worked and the TDI was averaging 55MPG. After changing the timing belt it stopped working, just 3 dashes and the MPG plummeted to an average of 45MPG. The sender unit was disconnected when I removed the bumper so I checked it, still no fix.
Ran with it for a while. I'd not used the car for a few days and the battery is getting weak, went to start it and the voltage dropped so low that the trip computer, MPG etc., all reset.......and low and behold, temperature gauge started working again! MPG now back up to around 54/55MPG average.
I'm suprised that it has made such a difference.