Coolant Temperature Below Thermostat Regulating Temperature
What it means
The engine is taking too long to reach normal operating temperature, or never gets there. The usual cause is a thermostat stuck open, which lets coolant flow through the radiator and cool down before the engine warms up.
Symptoms
- Temperature gauge stays low or rises very slowly.
- Slightly higher fuel consumption.
- Weak cabin heat in cold weather.
- Check engine light on.
Possible causes
- Thermostat stuck open or opening too early.
- Coolant temperature sensor reading too low.
- Radiator fan running constantly due to a control fault.
- Low coolant level.
- Thermostat with the wrong temperature rating installed.
How to diagnose and fix
- Check the coolant level with the engine fully cold.
- Monitor engine temperature with a scan tool and compare it to the dash gauge.
- Confirm the radiator fan is not running when the engine is cold.
- Replace the thermostat with one of the correct rating.
- Replace the temperature sensor if its readings are implausible.
Repair cost
A thermostat or sensor is usually inexpensive, but labor rises on engines where the thermostat is hard to reach.
Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Frequently asked questions
Is a cool-running engine a problem?
Yes. A cold engine uses more fuel, wears faster and prevents the computer from running some self-tests. It is not an emergency, though.
Can I just remove the thermostat?
Do not. The thermostat is essential for temperature control, and removing it causes the same problem or worse. Also, never open the radiator cap while the engine is hot.
This guide is for information only and does not replace an inspection by a qualified technician. If the fault involves brakes, steering, engine temperature, electrics or refrigerant, do not attempt the repair yourself.
Report a mistake
Spotted something inaccurate? Tell us and we will review and correct it.