P0128
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Powertrain · Generic (all makes)System: Fuel Delivery + Emissions
Safe to drive short term. Book a repair.
Likely causes
- Thermostat stuck open
- Faulty coolant temperature sensor
- Low coolant level
What to check first
Most often a thermostat stuck open: the engine never reaches operating temperature, which hurts fuel economy and heater output.
Typical repair cost
$150 – $400
Independent-shop parts and labor for the usual fix. Your price depends on the actual cause and the vehicle.
Next step
General information, not a diagnosis of your vehicle. Repair costs are typical shop ranges, not quotes.