With your coolant reservoir empty, your coolant system can’t draw in extra coolant when it needs it. This can lead to overheating. You can top up the reservoir, but you also need to identify where that coolant went.

If the radiator is full but the coolant reservoir is empty, it’s likely that you have a blockage in your system. This can be caused by a number of things, including: A build-up of sediment or rust in the radiator A broken water pump A blocked thermostat If you suspect that your car has a blockage, take it to a mechanic as soon as possible.