P0128: Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)

Short answer: P0128 means “Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)”. It is a standard OBD-II code, so the basic meaning is the same on every car. Below is what it means on each of the 5 cars we cover, with a link to the full page for each car.

Standard OBD-II code Same meaning on all cars (SAE J2012).

P0128 on each car

Symptoms

1. Check engine light on
2. You will likely not notice any drivability problems other than the
3. Illumination. In some cases you may experience symptoms such as
4. Engine takes longer to warm up
5. Engine temperature drops while driving at higher speeds
6. A photo of a typical thermostat

Likely causes

1. Low engine coolant level
2. Leaking or stuck open thermostat (likely)
3. Faulty cooling fan (running too much)
4. Faulty
5. Coolant temperature (ECT) sensor
6. Faulty intake air temperature (IAT) sensor

What to check, in order

1. Read the code and freeze-frame data with an OBD-II scanner
2. Check the connector and wiring of the part named in the code
3. Test the part with scanner live data or a multimeter, using the values in the repair manual
4. Fix the cause, clear the code and test-drive

Causes and checks above are the general meaning. The exact test steps and values differ by car: open your car’s page.

All fault codes