Overheats when idling/cooling fan won't switch on - NA (MKI)

Most likely cause of this is a problem with cooling fan switch. First of all you need to locate & disconnet the fan switch connector which is a single wire grey plug located on top of the thermostat housing. Now switch the ignition switch on and keep all objects and body parts clear of the radiator fans then using a paper clip short the wire to ground on a clean part of the thermostat housing. The fan should start up (on cars with A/C, only the passenger side fan should run) this shows the fault lies with the switch.

The switch simply unscrews from the top of the housing & the new one screws in, replace the O ring & tighten to 5.9-8.8Nm