P0128: Coolant thermostat (coolant temperature below regulating temperature)
The engine takes too long to reach normal temperature.
Updated
What P0128 means
A thermostat keeps coolant out of the radiator until the engine is warm, so the engine heats up quickly. The engine computer expects the coolant to reach a set temperature within a certain time, depending on the conditions.
If it takes too long, the computer stores P0128. A common reason is a thermostat stuck partly open, so the coolant is cooled in the radiator all the time.
Read My Car’s live data shows the coolant temperature, so you can watch how quickly it rises.
Can I keep driving?
Likely OK to drive
Likely OK to drive for now. Have it fixed before the next emissions inspection (APK, MOT, TÜV, contrôle technique).
General guidance based on the code alone, not on the car’s actual condition. If in doubt, stop and ask a mechanic.
Check the coolant level first, with the engine cold: low coolant is one of the causes, and a cooling system short of coolant can overheat.
Symptoms
- The temperature gauge stays low or takes a long time to rise, on cars whose gauge shows it.
- The heater stays lukewarm, especially in cold weather.
- Higher fuel use.
Common causes
- Thermostat stuck open. It lets coolant through the radiator before the engine is warm.
- Faulty coolant temperature sensor. It reports a lower temperature than the real one.
- Low coolant level. With too little coolant around the sensor, it can read wrong. Top up only when the engine is cold.
Typical fixes
- With the engine cold, check the coolant level and top it up with the coolant specified for your car.
- Watch the coolant temperature in live data during a drive. If it rises very slowly, or drops while driving, the thermostat is the usual suspect.
- If the temperature looks normal but the code comes back, have the coolant temperature sensor and its wiring checked.
- Clear the code after the repair and drive normally.
Questions
Can I drive with P0128?
Likely, for now: the app’s advice is that it’s likely OK to drive, and to have it fixed before the next emissions inspection. Check the coolant level first, with the engine cold.
Why does P0128 often show up in winter?
In cold weather the engine loses heat faster, so a thermostat that’s stuck open has a bigger effect, and the engine is more likely to miss the expected warm-up time.