P0128 – Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
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Symptoms
- High idle on cold start
- Poor heater performance
- Engine takes long to warm up
- Reduced fuel efficiency
Causes
- Stuck-open thermostat
- Low coolant level
- Faulty coolant temperature sensor
- Cooling fan stuck on
Diagnosis
- Check coolant level
- Monitor ECT sensor data
- Verify thermostat operation
- Inspect cooling fan behavior
Repair
- Replace thermostat
- Refill coolant
- Replace ECT sensor if faulty
- Repair cooling fan control issues
