My 2003 MCS started to overheat at idle yesterday, as I was sitting in the driveway listening to the end of a radio show. When I turned the engine off, the cooling fan didn't come on. There was plenty of coolant in the expansion tank, so I think the fan is the problem.

I've been trying to figure out if it is the fan itself or the relay that has gone. To test this, I turned on the aircon, to see if that would activate the fan. The fan didn't come on so I'm assuming it is the fan itself. Any opinions?

Hi ya might be the temp sensor for the fans. Think is around the water pump thermostate area.
Check your fuses as well if you have not already

I didn't think of the fuses. Thanks marque!

The fuse looked pretty good. I switched it just in case, but still haven't got the fan working. Is there any way to test the thermostat?

Here's my next question, the temp gauge on the dash went up when the car overheated, so some temperature gauge is working. Is it the same one that controls the fan, or is there a separate one?

I wouldnt imagine its the temp sensor if the fan doesnt work with the air con. Test the power at the fan itself when you have the air con on..also disconnecting the coolant temp sensor will bring on the fan as a fail safe, but this will also bring on a warning light...

water pump making any unusual noise? could be you just need a thermostat also. seems like when your crank damper is going the mini will overheat and also drop into limp mode.