Q: My car is having a high idle - I replaced the coolant temperature sensor and now the fans don't work

This may be related to a dirty or faulty idle air control valve. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine. This valve is controlled by the vehicle’s computer which will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage.

When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed. This is all done by the idle air control valve. When the engine RPM drops below the normal range of 750-800 RPM (for most cars), this may indicate a dirty or faulty idle air control valve. In cases where this valve is dirty or failing, your vehicle may stall instead of slowing to an idle after the release of the throttle. You may also want to check the cooling fan relay switch. As you may know, this is the switch that controls the operation of the fans based upon input data from the coolant fan temperature sensor which you have just replaced. When this relay is not working properly, this may cause the fans to work intermittently or in some cases not at all. I would recommend having an expert from YourMechanic come to your location to diagnose and repair your idle air control valve.