P0125 GMC Sierra 1500
Insufficient Coolant Temperature for Closed Loop · Powertrain Code · Medium · Repair: Moderate
What is P0125?
The engine is not reaching operating temperature quickly enough for the ECM to enter closed-loop fuel control. Usually caused by a stuck-open thermostat.
When the GMC Sierra 1500 triggers code P0125 (insufficient coolant temperature for closed loop), it typically points to an issue in the powertrain system. Below you'll find the most common causes, symptoms, and repair options specific to the GMC Sierra 1500.
This repair is rated moderate, meaning some experience and specialty tools required.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your GMC Sierra 1500.
Common Causes on GMC Sierra 1500
- Thermostat stuck open (very common on GM vehicles with Dex-Cool coolant)
- Failed ECT sensor reading lower than actual temperature
- Low coolant level preventing engine from reaching operating temp
- Cooling fans running continuously due to a relay or sensor fault
Symptoms
- Check Engine Light on
- Engine takes unusually long to warm up
- Heater blows lukewarm air in cold weather
- Poor fuel economy due to extended open-loop operation
Notes for GMC Sierra 1500
Dex-Cool coolant systems in GM vehicles are prone to thermostat gasket degradation — always replace the thermostat gasket/seal when replacing the thermostat.