The water temperature relays coolant temperature back to the ECU.  The sensor is located on the back of the water pump housing, right below the alternator.  Keep in mind there are several water temp sensors and switches used throughout the engine and cooling system, but the one located behind the water pump housing is the sensor used by the ECU for fuel/ignition timing correction.

Total failure of the water temp sensor will cause the ECU to fall into a fail-safe mode - the car will run very rich.  The car will still run with a bad water temp sensor.

Possible effects:
  • Rich mixtures
  • Lower MPG
  • Sluggish throttle response

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