A lot of people have posted on the subject of coolant sensors. I had slightly different symptoms to the norm - intermittent poor starting when half-warm, slight smell of petrol on cold starts and intermittent engine or ESP warning lights (or both!). A fault code reader produced two different codes over a couple of weeks (resetting the codes each time) - coolant temp too high and implausible inlet temperature (pointing to the inlet manifold sensor). The temperature gauge always appeared normal and the car was normal when warm. A closer watch of the temperature gauge showed that it fluctuated up and down in the bottom quarter of the scale when warming up from a cold start. Thought I'd try the coolant temp sensor and yes - everything is now ok!

A few possibly useful points: if the connector to the sensor is pointing down so that you can't easily "unlock" it from the sensor you can grab the sensor body and rotate the whole thing so the connector points up and it's therefore easy to release with a screwdriver; if the wiring to the sensor plug is bent back and held onto the plug with a cable clip then it's easier to snip this off first as it gives you more slack in the cable and makes it easier to get the plug off; the old o-seal tends to stay in the housing (not come off with the sensor) so remember to recover it.

I split open my faulty sensor and found that the problem was water ingress - there is an o-seal inside between the metal part and the grey plastic and the roll or crimp, if you prefer, of the metal part over the plastic is supposed to provide the compression on the o-seal to make a hermetic seal; in my view this sensor had not been manufactured correctly (insufficient pressure on the seal) and so allowed water in. Once inside you can imagine this water is a vapour when warm and so has no effect when the sensor is hot but when it's cold it condenses and changes the resistance if it sits on the right bit - a bit of fiddling with a multimeter confirms this. Ok, now I'm sounding a bit sad, but at least it explains why these things appear to behave erratically rather than just failing open or closed circuit.

Finally, my local Skoda dealer (of whom I'm not a great fan!) had enormous trouble working out which sensor I required - I recommend giving them the chassis number when ordering so they can double check!

Thanks for the tips.

Which model Fabia do you have?

Oh yes, sorry - I have an '02 Fabia auto.