P0128 – Coolant Thermostat (Coolant Temperature Below Regulating Temperature)
Description
The engine does not reach normal operating temperature within the expected time, often due to a stuck-open thermostat.
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Compare Diagnostic ScannersSymptoms
- High idle on cold start
- Poor heater output
- Long warm-up time
- Reduced fuel economy
Causes
- Stuck-open thermostat
- Low coolant
- Faulty ECT sensor
- Cooling fan stuck on
Diagnosis
- Check coolant level
- Monitor ECT data
- Verify thermostat operation
- Inspect cooling fan
Repair
- Replace thermostat
- Refill coolant
- Replace ECT sensor
- Repair fan control
