CHECK ENGINELIGHT CODE Look up a code
OBD-II CODE / Cooling

P0128 · Coolant below thermostat regulating temperature

The engine coolant warmed more slowly or stayed cooler than the computer expected for the conditions.

Sources checked · Independent technician review pending · Report an error

Start here

Record the temperature data and check coolant level only when the engine is cold, using the owner’s manual.

Look at the warm-up curve

A stuck-open thermostat is one possibility. Ask whether the coolant temperature reading made sense at a cold start and how it changed as the engine warmed. Coolant level and temperature-sensor readings can change the diagnosis.

If the car is overheating

Treat a high-temperature warning or visible steam as a separate urgent symptom. Pull over safely and follow the owner’s manual. Leave the pressurized cooling system closed while it is hot.

A vehicle-specific example

Stellantis STAR case S2118000009 describes checking the coolant temperature sensor before replacing the thermostat when the initial cooling-system checks find no issue. Match the test procedure to the vehicle.

Sources

Keep reading