Engine and gearbox (P)
P0128: Coolant Thermostat Temperature Below Regulation
This code indicates that the engine is taking too long to reach its normal operating temperature. It suggests a problem with the thermostat or the coolant system.
Checked by a licensed car technician · last reviewed 29 September 2026
Symptoms
- Engine takes longer to warm up
- Poor fuel economy
- Heater not working efficiently
- Check engine light on
Common causes
- Thermostat stuck open
- Coolant temperature sensor bias
- Low coolant level
- Faulty wiring or connections
How to check it yourself
- Check coolant level
- Inspect thermostat operation
- Test coolant temperature sensor
- Examine wiring and connections
Can I keep driving?
The car can still be driven, but prolonged driving with this issue may lead to reduced fuel efficiency.
General guidance, not a professional inspection. Confirm faults with measurements before buying parts, and use a qualified mechanic for brakes, steering, fuel leaks, airbags, and other safety-critical work. Terms and disclaimer