Code
P0128
Cooling
Coolant thermostat below regulating temperature
The engine never reaches normal temperature in the expected time, usually a thermostat stuck open.
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Most likely causes
- Thermostat stuck open
- Low coolant level
- Faulty coolant temperature sensor
Symptoms you may notice
- Check engine light
- Engine warms up slowly
- Weak heater output
Checks to run
- Verify coolant temperature reaches about 90 °C
- Test thermostat opening temperature
- Check coolant level
Tools you may need
OBD-II scanner
Infrared thermometer
Always diagnose before replacing. Codes share symptoms; confirm the cause with measurements before buying parts.