Do I have enough evidence to convince a dealer that the coolant temperature sensor on my Ibiza FR TDI is faulty? The symptoms are: Fuel economy has not quite improved as much as I'd expect now that the ambient temperature has picked up after the winter. It's maybe 4 or 5 mpg down on what I'd expect - particularly for short/medium journeys. Glow plug light still stays on for at least 5 seconds when starting from cold, even when ambient temperature is above 10C. After waiting for glow plug light to go out, the engine starts with slightly more revs than normal. It revs to about 1400 rpm before settling back down to normal idle speed. Warm starts are normal - GP light goes straight out and engine starts at normal revs. If I plug in VAG-COM before starting and the engine has been off for many hours, the coolant temperature sensor often disagrees with the ambient temperature. For example, a few days ago it was 15C but the coolant temperature said minus 1.8C. Coolant temperature reads a sensible figure when the engine is hot - about 86C or so. The other night I was running the engine in Group 11 Basic Settings to exercise the VNT mechanism (I do this whenever I've been stuck in traffic jams in the last few days), and the dashboard temperature gauge was moving up and down. A couple of weeks ago I had a fault code: "17664 Engine Coolant Temp Sensor (G62): Open or Short to Plus P1256 -35-10 - - - Intermittent". But foolishly I reset this code and it hasn't come back, so I've lost the evidence. Has anyone had experience of a faulty coolant temperature sensor - are the dealers likely to accept that there's a problem, even though the performance of the engine is normal (except perhaps the marginal mpg degradation)?