I have a 99 vectra. The fan isn’t coming on when the car gets hot, but when I take the plug out the side of the rad and short it out it works fine.

Does this mean I just need a new temp sensor switch or thermostat – or both? How can I tell which I need?

Thanks

Do some checking first;

1) Find out where the thermostat is in the engine. Look for the hose leading from the thermostat to the radiator. Start the engine and hold the hose. Does it stay cold for a few minutes after starting the engine? and then rapidly get hot? (as the thermostat opens). If so then the thermostat is opening and functioning correctly.

2) Remove the temp’ sensing switch, suspend it in some cold water in a saucepan and put it on the stove. Connect a multi-meter across the switch and light the gas under the pan. Check it switches just before the water boils. Most switch between 88 and 98 Celsius.

Whichever test doesn’t give the result you expect indicates which part needs replacing 😀

Neither. The temperature sensor switch is for the temperature guage in the car. The thermostat opens up the rest of the cooling system when the engine gets up to temperature. The switch on the side you are shorting out is the cooling fan switch. When the water in the radiator gets to a certain temperature it turns the fan on.

If you don’t know when the thermostat was last changed i’d change that at the same time as well. They don’t cost a lot and if they are faulty can cause a lot of expense. Remember to top up the radiator and bleed it.