OBD-II code · Powertrain (generic)
P0128 Coolant Thermostat Below Regulating Temperature
The engine has been running long enough to reach operating temperature, but coolant temperature stayed below the thermostat's regulating value. Almost always caused by a thermostat stuck open. Costs fuel economy and increases cold-engine emissions.
Is it safe to drive with P0128?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyThermostat stuck open
- Less likelyCoolant sensor reading low
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Often, with basic tools
- Typical labour
- 0.7–3 h
- Emissions test
- Likely to fail
- After the fix
- A drive cycle is needed before the light stays off
- Parts involved
- Thermostat, Coolant Temperature Sensor
Related codes
- P0125Insufficient Coolant Temperature for Closed Loop Fuel Control
- P0126Insufficient Coolant Temperature for Stable Operation
- P0127Intake Air Temperature Too High
- P0129Barometric Pressure Too Low
- P012ATurbocharger/Supercharger Inlet Pressure Sensor A Circuit
- P012BTurbocharger/Supercharger Inlet Pressure Sensor A Circuit Range/Performance
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.