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.
General code reference. Confirm the definition and diagnostic procedure for your vehicle's year, model and engine. A stored code alone does not identify a failed part.
Affected components
- Thermostat
- Coolant temperature sensor
Possible causes to check
These are possible checks, not a ranked diagnosis. Follow the vehicle manufacturer's test procedure before replacing parts.
- Thermostat stuck open
- Coolant sensor reading low
Diagnostic notes
- The malfunction indicator lamp may illuminate
- This code can affect emissions
- A drive cycle may be required after repair
Sources
Check each reference's vehicle and model-year scope. General background links do not establish which fault is most likely on your vehicle.
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.