P0128
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Low· Fix when convenientDIY friendlyPowertrainGeneric, all makes
What it means
The engine is not reaching its normal operating temperature in the time the ECU expects. The thermostat is almost always the cause: it is stuck open, so coolant flows through the radiator before the engine has warmed up.
Is it safe to drive?
Safe to drive. Fuel economy and heater output will be lower, and engine wear increases slightly when it runs cold. Fix when convenient.
Symptoms
- Check engine light on
- Temperature gauge reads low or takes a long time to rise
- Heater not getting hot
- Lower fuel economy
Probable causes
Most likely first
- 1Thermostat stuck open (most common)
- 2Faulty coolant temperature sensor reading low
- 3Low coolant level
- 4Cooling fan stuck on
How to diagnose it
Work through these in order before buying parts
- 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
- 2Check coolant level.
- 3Watch coolant temperature on live data on a drive from cold; compare with an infrared thermometer at the thermostat housing.
- 4Check whether the cooling fan runs when the engine is cold (it should not).
- 5Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.
Common fixes
- Replace the thermostat (use the correct temperature rating and a new gasket)
- Replace the coolant temperature sensor
- Repair the fan control
This information is general guidance for all makes. Exact causes, specifications and procedures vary by vehicle, so confirm with your service manual or a qualified mechanic before replacing parts.