fault code guide
P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
This code means your engine is not warming up to its proper operating temperature within the expected time. You can still drive, but the car will use more fuel and the cabin heater will stay lukewarm.
How it shows up: The amber engine management light will glow steadily on the dashboard, and a diagnostic scanner will read code P0128. You might also notice the cabin heater blowing lukewarm air and the dashboard temperature gauge staying near the cold mark.
Updated 9 November 2026
Can I keep driving?
Service soon
Driving with an engine that runs too cold uses significantly more fuel, increases engine wear and will eventually clog emissions components.
What to do right now
- 1Check your dashboard temperature gauge to confirm the engine is running cold rather than overheating.
- 2Make sure the engine management light is steady amber; if it begins flashing, pull over safely and switch off the engine.
- 3Plan to have the car inspected soon to prevent unnecessary fuel waste and carbon build-up.
Likely causes
Most likely first.
- Likely
Thermostat stuck open
The thermostat valve has jammed or degraded, allowing coolant to circulate constantly through the radiator. This prevents the engine from reaching its ideal operating temperature.
- Possible
Faulty engine coolant temperature sensor
The sensor or its wiring can send a falsely low temperature reading to the engine computer. Even if the engine is warm, the computer believes it is still cold.
- Possible
Low engine coolant level
An air pocket from low coolant can leave the temperature sensor dry, preventing it from registering actual engine heat accurately.
- Rare
Cooling fan running continuously
A faulty fan switch or relay can keep the radiator cooling fan spinning constantly. This over-cools the coolant as you drive, especially in cold weather.
What you can check yourself
Park somewhere safe and level, engine off, before you start.
- 1
Check the coolant level
Wait until the engine is completely cold, open the bonnet, and check the plastic expansion tank. The fluid should sit between the minimum and maximum markings without opening the cap.
- 2
Watch the dashboard temperature gauge
During a normal journey of fifteen minutes, note whether the temperature needle climbs to the centre or stays low. If it struggles to rise or drops on the motorway, the thermostat is likely stuck open.
- 3
Test cabin heater temperature
Turn the cabin heater to maximum heat with the fan on medium once the car has driven for ten minutes. If the air stays lukewarm or cool, hot coolant is not circulating properly through the heater core.
- 4
Inspect the temperature sensor wiring
Look near the thermostat housing for the electrical plug connected to the coolant sensor. Check that the connector is pushed on firmly and that the wires are not frayed, cracked or soaked in oil.
Rough cost to fix
Rough estimateTypical prices in Ireland (€) and the UK (£), from an independent garage up to a main dealer. Thermostat replacement costs vary widely depending on whether your car uses a simple mechanical valve or an electronically controlled thermostat integrated into a complete plastic housing.
Thermostat replacement
- Do it yourself
- €30–90£25–80 in the UK
- At a garage
- €140–320£120–280 in the UK
Coolant temperature sensor replacement
- Do it yourself
- €20–60£15–55 in the UK
- At a garage
- €90–190£80–170 in the UK
Coolant flush and system bleed
- Do it yourself
- €25–60£20–50 in the UK
- At a garage
- €80–160£70–140 in the UK
When to go to a garage
- The temperature gauge begins rising towards the red hot mark or the high coolant temperature warning illuminates.
- You spot visible coolant pooling under the car or dripping from hoses and housings.
- The cabin heater continues blowing cold air despite the coolant reservoir being full.
- Replacing the thermostat or clearing the code fails to turn off the engine management light.
Questions drivers ask
Can I keep driving my car with code P0128?
Yes, you can safely complete your journey and drive locally for a short period. However, because the engine runs below its designed temperature, the computer injects extra fuel to try to warm it up. Prolonged driving will waste petrol or diesel, cause carbon build-up in the cylinders, and eventually poison the catalytic converter.
Will clearing code P0128 make it go away for good?
Clearing the code with a scanner only wipes the computer memory; it will not repair the hardware fault. As soon as the engine control module detects that coolant temperature rises too slowly over two driving cycles, the light and code will return. The underlying stuck thermostat or faulty sensor must be renewed first.
Will code P0128 cause my car to fail the NCT or MOT?
In the UK, a steady engine management light caused by P0128 is an automatic MOT failure on petrol cars registered from July 2003 and diesels from July 2008. In Ireland, the engine light alone results in an Advisory Pass at the NCT, provided the cold-running engine does not cause exhaust emissions to exceed legal limits.
How is P0128 different from code P0125?
Code P0128 specifically indicates that the engine coolant temperature has not reached the thermostat regulating threshold within a set driving time. Code P0125 means the coolant has not reached the temperature required for closed-loop fuel control, which is often tied directly to faulty sensor feedback rather than a mechanical thermostat fault.
Test rules here are for the National Car Test in Ireland (NCT) and the MOT in the UK. Elsewhere, check the regulations of your own country or state before going for a vehicle test.
The warning light you may see
Related fault codes
Get an answer for your own car
Tell the AI Technician your make, model and what you are seeing. It works out the likely cause, the tests and the fix for your car.
Free with an AutoQ Pro account. Answers in about ten seconds.
General guidance for most cars, not a diagnosis of yours. Details differ by make, model and year - check your handbook, and if in doubt, don't drive.