A "check engine" light coupled with poor gas mileage could mean dozens of different things are wrong with your engine. But if you take it to get diagnosed and the mechanic says that the problem is your coolant temperature switch, then you've got yourself a problem. Thankfully, this is a problem that is easy to fix, but it can lead to more serious issues for your engine if it is left unattended. The most common issue with engines that have a broken coolant temperature switch is overheating. It is not a forgone conclusion that you will overheat, but one of the functions of this part is the operation of the engine cooling fan. If that doesn't turn on when it needs to be turned on, then the radiator won't get the help it needs to dissipate the heat from the coolant. It also tells your engine's ECU how hot the engine is, and it is supposed to adjust the air/fuel mixture and timing to compensate for it. That is why a broken coolant temperature switch could possibly result in poor gas mileage.
Much like other sensors in modern engines, the coolant or water temperature switch receives a constant supply of power from the engine control unit. As hot coolant passes over the sensor element, the resistance changes slightly and less power returns to the ECU. This is called voltage drop, and it is measured by the computer to determine the temperature of the coolant. When it exceeds a certain threshold, the coolant temperature switch will turn the cooling fan on. That will draw more air in past the fins of the radiator, and hopefully reduce the temperature of the coolant inside its core. This can most commonly be heard when your car is sitting at idle for too long.
Replacing one of these switches is very easy, and only takes minutes. All you have to do is find where your existing coolant temperature switch is mounted. They are usually somewhere near the thermostat housing or water pump. Once you have found it, disconnect the wire harness from it and unscrew it. You can now install your new water temperature switch and attach the wire harness connector. It is safest to perform this with your battery disconnected.
The coolant temperature switch in your car is not a terribly expensive part... unless you get it from the dealership. At Car Parts Discount, we recognize that you want to save money wherever you can, and by paying ten times what something is actually worth you are not accomplishing that goal. That's why we carry great coolant temperature switches from the top manufacturers in engine electronics and sell the parts at amazingly low prices. There's no need to drop a full day's pay on something that should cost the price of a meal. Order your parts at CPD from now on and watch the savings pile up.