Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
What it means
The engine computer watches coolant temperature rise after starting and expects it to reach the temperature the thermostat regulates within a set time. This code means the temperature stayed lower than expected; by far the most common cause is a thermostat stuck open that sends coolant to the radiator too early.
Related lights
Dashboard lights that may come on with this code.
Symptoms4
- Check engine light on, often with no other symptoms in hot weather.
- Temperature gauge lower than usual or dropping during highway driving.
- Weak cabin heating in cold weather.
- Slightly higher fuel consumption.
Possible causes5
- Thermostat stuck open or not sealing fully.
- Coolant temperature sensor reading lower than the real temperature.
- Low coolant or air in the system.
- Radiator fans running constantly from a stuck relay or control fault.
- Inaccurate intake air temperature reading, which some systems use to calculate the expected temperature.
How to diagnose5
- Check the coolant level with the engine cold.
- Watch coolant temperature on a scan tool from a cold start; if it stays below normal operating temperature or drops on the highway, the thermostat is likely open.
- Carefully feel the upper radiator hose while warming up; it should stay cool until the thermostat opens.
- Compare the sensor reading with an external thermometer.
- Make sure the fans are not running while the engine is cold.
How to fix5
- Top up the coolant and bleed air from the system.
- Replace a stuck relay if the fans run constantly.
- Replace the coolant temperature sensor if it is proven to read low.
- Replace the thermostat with a part matching the manufacturer opening temperature.
- Clear the codes and drive until the self-tests complete, and confirm the code does not return.
Repair cost
In Saudi riyals; the highlighted part is the expected range for this fault.
In the Saudi market: SAR 80–600 (≈ USD 21–160)
Our prices come from the Saudi market, converted at a fixed rate (SAR 3.75 per USD). Prices in your country may differ a lot, so we show the cost level.
The cost is low when low coolant or the sensor is to blame, and rises when the thermostat is replaced, especially on engines where it 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.
Prices last updated: September 2026
Frequently asked questions
Why does this code appear in summer despite the heat?
Because a thermostat stuck open makes the engine run cooler than intended even in hot weather, especially on highways where airflow cools the radiator strongly.
Can I remove the thermostat instead of replacing it?
No. Without a thermostat the engine runs cold, which raises fuel use and wear and brings the code back, and it can upset overall temperature control.
Is this code dangerous?
Not immediately, and you can keep driving, but fix it soon to protect fuel economy and engine life.
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