P0128
Get Serviced SoonFix: Medium — get serviced in 1–2 weeksCoolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature) — Engine is not reaching proper operating temperature, often a stuck-open thermostat.
Dashboard light triggered
Check Engine LightWhat to do
Replace the thermostat. A stuck-open thermostat causes poor fuel economy and excess wear. Heater may blow cooler than normal.
Common causes
- Stuck-open thermostat
- Wrong thermostat
- Coolant temperature sensor fault
Estimated repair cost
$100–$400
Exact cost depends on the cause, your vehicle, and labor rates in your area.
What to tell your mechanic
Use this script so you sound informed and get a clear diagnosis before agreeing to repairs:
“Code P0128, coolant below thermostat temp. Can you confirm the thermostat is stuck open and replace it? Heater hasn't been as warm lately.”
About the Check Engine Light
Code P0128 triggers the Check Engine Light. Indicates an issue with the engine or emissions system. Could range from a loose gas cap to a serious engine misfire.
Read full Check Engine Light guideFrequently asked questions
- What does OBD code P0128 mean?
- Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature) — Engine is not reaching proper operating temperature, often a stuck-open thermostat.
- What dashboard light does P0128 trigger?
- P0128 triggers the Check Engine Light.
- How serious is P0128?
- Severity: yellow. Fix priority: Medium — get serviced in 1–2 weeks.
- How much does it cost to fix P0128?
- Repair costs typically range from $100–$400. The exact cost depends on the root cause and your vehicle.
Related OBD codes (same dashboard light)
These codes also trigger the Check Engine Light: