P0128low severityPowertrain
Coolant Thermostat Below Regulating Temperature
on 2026 GMC Hummer EV SUV 3X Tri Motor AWD · AWD
What this code 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
Repair notes
Replace thermostat.
Other Powertrain codes on this vehicle
P0070
Ambient Air Temperature Sensor Circuit
P0071
Ambient Air Temperature Sensor Range/Performance
P0072
Ambient Air Temperature Sensor Circuit Low
P0073
Ambient Air Temperature Sensor Circuit High
P0115
Engine Coolant Temperature (ECT) Sensor 1 Circuit
P0116
ECT Sensor 1 Circuit Range/Performance
P0117
ECT Sensor 1 Circuit Low Input
P0118
ECT Sensor 1 Circuit High Input
P0119
ECT Sensor 1 Circuit Intermittent
P0125
Insufficient Coolant Temperature for Closed Loop Fuel Control