After ignition, the computer waits for the coolant to reach operating temperature for a certain time. If not enough, write P0128.
Low. The car is driven, but the consumption increases, the steam blows cool, and the cold engine wears out faster.
All liquid cooled engines. Thermostat, temperature sensor, fan.
What does P0128 mean
The thermostat keeps the fluid in the small circuit until the engine warms up, and only then turns on the radiator. If it is stuck open, the fluid cools all the time and the engine does not 85–95 °C, especially in the winter and on the highway.
Symptoms
- Temperature gauge stays low or drops when driving
- The steam blows cool
- Higher fuel consumption
- Lamp "Check Engine", most often during the cold months
| Can it be driven?
Yes. It's not urgent, but don't leave it for long - a cold engine runs a richer mixture, collects deposits and wears the oil faster. With diesels, this also interferes with the regeneration of the DPF.
Common reasons
- Thermostat stuck open — the most common reason.
- Faulty temperature sensor — shows lower than the real one (see P0116).
- The radiator fan runs constantly — stuck relay or sensor.
- Coolant level low — the sensor "sees" air.
How we diagnose it
- We monitor the temperature in real time from a cold start.
- We feel the upper hose of the radiator - if it heats up immediately, the thermostat is open.
- We compare the readings of the sensor with real temperature.
- We check the level and the fan.