P0128: coolant below thermostat temperature
The engine took too long to warm up, or never reached the temperature the thermostat is meant to hold.
SeverityLowLittle risk to the car, but the fault stays until it is repaired.
Can you keep driving?Safe to drive, book a checkWhat does P0128 mean?
The engine computer works out how fast the coolant should warm up from the outside temperature, the time running and the load. If the coolant stays below the expected regulating temperature for too long, it stores P0128. The commonest reason by far is a thermostat stuck partly open, which sends coolant through the radiator all the time.
What causes P0128?
- A thermostat stuck open, or opening too early
- A faulty coolant temperature sensor reading low
- A radiator fan that runs all the time
- Low coolant level
What to check first for P0128
- Check the coolant level with the engine cold
- Watch the temperature gauge, or a scan tool, on a drive: it should rise steadily and then hold
- Check that the radiator fan is not running on a cold engine
- Compare the coolant sensor reading with the engine's actual temperature
Can you drive with P0128?
It is not urgent and the car is safe to drive, but fuel consumption rises, the heater blows cooler air and the engine wears faster when it never warms up properly. Book it in.
In states with an OBD-II emissions inspection, a car with the check engine light on fails the test until the fault is fixed.
Codes on this page
- P0128Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Related codes
Checked on · Rafael Soares
These are the generic meanings every manufacturer shares. Each brand adds its own detail, so your car's handbook, its service information and a qualified mechanic have the last word. OdoKeep has no connection with any manufacturer, and the app does not read codes from the car.
