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Coolant Temperature Sensor
Some Like it HOT! But your vehicle's engine does not like it TOO HOT. The coolant temperature sensor sends a signal to a gauge, light, and/or your computer indicating the radiator fluid temperature. This can be critical in preventing an overheating situation.
Modern cars and trucks may have a few different coolant temperature sending units. The most important coolant temperature switch is the one that tells the computer how hot the Anti-Freeze is. A radiator fluid temperature sensor that sends a signal to the computer is best diagnosed through your vehcle's On Board Diagnosis System. In a modern vehicle the computer uses the signal from this sensor to determine critical factors such as air/fuel mixture, and timing advance.
If your vehicle has a temperature gauge you may diagnose the antifreeze temperature switch. Locate the radiator fluid Switch that sends the signal to the temperature gauge. Have a helper turn the key to 'run' or 'on' and watch the temperature gauge. When you unplug the wire from the sensor, the gauge should go to its lowest position. Touch the end of the lead to a piece of iron (the engine block or a bracket). When you ground the wire the temperature gauge should go to its highest reading. If your gauge does not work correctly during normal running but responds as above, then your gauge is fine but your coolant temperature sensor is bad. With age these sensors can lose their calibration and give poor readings.
In older vehicles with a temperature indicator light, your system has built in checking mechanism. When you turn your key to the 'on' or 'run' position the light lights up indicating that the bulb works and that is there is a connection to the sending unit. Once you start your vehicle, the light should go off. If the light does not go off, when your engine is clearly cold, then there is a short in the circuit or your sending unit is bad. If the light fails to light when in the run or on position check the bulb and fuse (where applicable). If both the bulb and fuse are OK , your sending unit is to blame. Also sending units of this type should be replaced with age as they tend to be unreliable and may not tell you when the engine is too hot.
OBDII Check Engine Codes related to Coolant Temperature Sensors
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