P0128: Kühlmittelthermostat — Temperatur unter Regeltemperatur
Kühlmittelthermostat — Temperatur unter Regeltemperatur: Engine coolant did not reach target operating temperature (typically 82–90 °C / 180–195 °F) within the calibrated warm-up time and airflow integral.
| Trouble Code (DTC) | P0128 |
|---|---|
| Official SAE Definition | Kühlmittelthermostat — Temperatur unter Regeltemperatur |
| Vehicle Subsystem | powertrain |
| Driving Severity | LOW |
Observable Symptoms
- Heater blows lukewarm air in winter
- Slightly increased fuel consumption
- Temperature gauge stays below midpoint
Most Probable Root Causes
- Thermostat stuck open or leaking past rubber seal
- Low coolant level
- Faulty Engine Coolant Temperature (ECT) sensor reading artificially low
Step-by-Step Diagnostic Procedure
- Monitor live ECT sensor rise from cold start
- If upper radiator hose warms up immediately before 75 °C, replace stuck-open thermostat