Coolant temperature sensor fault

Symptoms

• Temperature gauge reads inaccurately (too high, too low, or stuck)
• Check-engine light on with a temperature sensor-related code
• Cooling fan behaving abnormally, either not coming on or running constantly
• Possible rough idle or poor fuel economy since the engine computer also uses this sensor

Likely causes

• CommonSensor internally failed or reading inaccuratelyThese sensors can drift out of accurate calibration or fail electrically with age.
• CommonCorroded or damaged sensor connectorThe electrical connector at the sensor is exposed to heat and moisture and can corrode over time.
• PossibleWiring damage between the sensor and the engine computerChafed or damaged wiring can interrupt or distort the signal even with a good sensor.
• PossibleLow coolant level affecting the sensor's contact with coolantSome sensor designs need to be surrounded by coolant to read accurately, and a low level can throw off readings.

What to do

1. Check the coolant level, since a low level can sometimes affect sensor readings on certain designs.
2. Get any check-engine light codes read with an OBD-II scanner, which will often point directly to the coolant temperature sensor circuit.
3. Check the sensor's electrical connector for corrosion or looseness if accessible.
4. Note any related symptoms like fan behavior or fuel economy changes that support a sensor issue versus a simple gauge malfunction.
5. Avoid ignoring an inaccurate gauge, since it can mask a real overheating problem if it's reading falsely low.
6. Have a mechanic test the sensor's actual resistance against known specifications to confirm the fault.
7. Replace the sensor once confirmed faulty, which is usually a straightforward and inexpensive repair.

Tools and parts

• OBD-II scanner

When to call a pro

See a mechanic if you're not comfortable testing the sensor's resistance yourself, since a wiring issue can sometimes mimic a sensor fault and needs to be ruled out.

Safety