P0128
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
Detailed Description
After the engine has been running long enough, the ECM has detected that the coolant temperature still has not reached the thermostat set operating temperature (typically around 90°C).
Possible Causes
Thermostat stuck in the open position
Coolant temperature sensor reading high
Insufficient coolant level
Radiator fan running continuously.
Solutions & Repair Guide
This is a very common fault code. First check if the coolant level is adequate; Use an infrared thermometer to measure the temperature difference between upper and lower radiator hoses to determine if the thermostat is opening properly; If the temperature difference is small and normal operating temperature is never reached, replace the thermostat; Rule out sensor faults.
Disclaimer: The information above is for reference only and does not constitute professional repair advice.
Actual causes may vary depending on vehicle make, model, year, and configuration. Always consult a qualified technician for accurate diagnosis.