P0128
PowertrainTemperature Sensorlow severityCoolant Thermostat Below Regulating Temperature
What P0128 Means
The engine coolant has not reached the expected temperature within a specified time after startup.
Symptoms
- Check Engine Light
- Poor fuel economy
- Heater not getting hot
- Engine running cool
Common Causes
- Stuck-open thermostat
- Missing thermostat
- Faulty ECT sensor
- Cooling fan stuck on
Diagnostic Steps
- Monitor ECT warmup curve
- Check thermostat
- Verify fan operation
- Test ECT sensor
Where to start: Replace thermostat.
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