As in there is another sensor in the top of the radiator? I know there is a thermostat on the side of the engine block under the timing belt cover, but I don't think it can be that as the point where the coolant is boiling is after the thermostat.

If it isn't the temperature sensor it could be wiring (including earth) fan relay, or the fan itself.

You can test the sensor by sticking it in very hot water and seeing the effect on resistance as measured with a multimeter. Fiddly and quite easy to scald yourself.

Intermittent problem on the Skywing, and I ended up making a breakout for the fan relay and bypassing the wiring loom from the sensor with new external wire.

Seems to be working so far, though my dodgy crimped connectors are probably vulnerable to damage.

The "intermittant problem" resulted in it hoiling over during both the previous 6-monthly emission testing sessions, straining the testers patience (which I definately need to conserve) and requiring retests.

I put two sets of "emergency " bypass wiring in to directly supply the fan, one under the bonnet and another operated from the cab

I forgot to remove or hide these on the test session yesterday, generating a certain amount of extracurricular noseying about on the part of the tester, as did my IKEA meat thermometer temperature gauge substitute, but they passed it anyway.

Edited by edlithgow on 01/05/2021 at 01:14

keep an eye on your temperature gauge, as coolant leaks from these is usual cause of scrappage