OBD-II · Engine and powertrain (P)
P0128
Coolant temperature below thermostat regulating temperature
What it means
The engine warms up too slowly or runs colder than expected.
Symptoms
- temperature gauge stays low or rises slowly
- poor heater output in winter
- higher fuel consumption
- engine warning light
Likely causes, most likely first
| No. | Cause |
|---|---|
| 01 | thermostat stuck open |
| 02 | low coolant |
| 03 | faulty coolant temperature sensor |
| 04 | cooling fan running too much |
| 05 | incorrect thermostat |
What to check, step by step
- 01Check the coolant level
- 02Watch the coolant temperature during a steady drive: if it sits well below normal operating temperature, the thermostat is likely stuck open
- 03Compare the sensor reading with an infrared thermometer to rule out a faulty sensor
- 04Check that the cooling fan is not running constantly
- 05Replace the thermostat with the correct temperature rating (on many modern cars it is electronically controlled and housed with the sensor)