Coolant temperature warning light

Stop the car now

Pull over and switch the engine off as soon as you safely can. Not in a few minutes, not at the next exit five miles down the road. Seconds matter here. A hot engine that keeps running can warp a cylinder head, blow a head gasket, or seize completely, and none of those are cheap or quick to fix. An overheat caught early is usually a simple repair. An overheat you keep driving through often is not.

What the light means

This is a temperature sensor telling you the coolant in the engine has gone past the safe range. The cooling system is supposed to keep the engine at a steady operating temperature. When this light comes on, that system has stopped doing its job, or there is not enough coolant left in it to do the job at all.

Common causes, most likely first

Low coolant is the most common reason, usually from a leak somewhere in a hose, the radiator, or a gasket. A stuck thermostat is another frequent cause — it fails shut and blocks coolant from circulating. A failed water pump will do the same thing, since it is what pushes the coolant around in the first place. A cooling fan that has stopped working, whether the motor or the sensor that controls it, is common in stop-and-go traffic or on hot days. Less often, a blown head gasket lets combustion gases into the cooling system, which pushes temperatures up fast and is more serious. A clogged radiator, from old coolant or debris, can also be the culprit.

Do not open the radiator cap or the coolant reservoir while the engine is hot. Pressurized hot coolant can cause a serious burn. Wait until the engine has cooled before checking anything under the hood.

What it usually takes to put right

A lot of these are straightforward — topping off coolant and fixing the leak that caused it, replacing a thermostat, or repairing a fan circuit. Those are routine jobs. A water pump replacement takes more labor since it is usually driven off the timing belt or chain. A head gasket job is a much bigger repair, and if the engine ran hot for a while before you stopped, there can be additional damage to check for. We can't tell which of these it is without looking at the car, and we won't guess a price over the phone. If the light comes on, get it towed or bring it in rather than risk driving it in, even a short distance.