Coolant Temperature Below Thermostat Regulating Temperature
Common Symptoms
- Check engine light on
- Heater takes longer to warm up
- Temperature gauge stays low
- Poor fuel economy
- Rich fuel mixture during extended warm-up
Most Likely Root Causes
- Thermostat stuck open or removed
- Faulty coolant temperature sensor (reading cold)
- Low coolant level
- Incorrect thermostat installed (too cold)
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Severity / Derate Risk
Low — no derate. Fuel economy and emissions affected.
Component Location
Thermostat in thermostat housing, typically where upper radiator hose meets engine. ECT sensor in coolant passage near thermostat housing.
Manufacturer Notes
Most common cause is thermostat stuck open. Thermostats are inexpensive — replace rather than test. Verify ECT sensor accuracy before condemning thermostat.
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