OBD-II code guide
P0128 Coolant Temperature Below Thermostat Regulating Temperature
P0128 usually means the engine is warming up too slowly or not reaching the expected operating temperature.
What it means
The computer expected warmer coolant by a certain time and driving condition.
Common symptoms
- Check engine light
- Poor heater output
- Lower fuel economy
- Temperature gauge stays low
- Long warm-up time
Likely causes
- Thermostat stuck open
- Low coolant
- Coolant temperature sensor issue
- Very cold weather edge case
- Cooling fan running too much
First checks
- Watch the temperature gauge after a cold start.
- Check coolant level when cold.
- Compare heater output to normal.
- Scan live coolant temperature if available.