P0128 Jeep Grand Cherokee
Coolant Thermostat Below Regulating Temperature · Powertrain Code · Medium · Repair: Easy
What is P0128?
The engine coolant is not reaching the expected temperature. Most commonly caused by a stuck-open thermostat, low coolant level, or a faulty coolant temperature sensor.
When the Jeep Grand Cherokee triggers code P0128 (coolant thermostat below regulating temperature), 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 Jeep Grand Cherokee.
This repair is rated easy, meaning basic tools, no lift needed.
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 Jeep Grand Cherokee.
Common Causes on Jeep Grand Cherokee
- Thermostat stuck open on 3.6L Pentastar V6 engine
- Thermostat housing cracked from thermal stress (known Pentastar issue)
- Coolant temperature sensor providing erratic voltage signal
- Low coolant from slow leak at water pump housing gasket
Symptoms
- Engine takes abnormally long to reach operating temperature
- Heater blows lukewarm or cold air
- Temperature gauge stays below normal or never reaches middle
- Slightly reduced fuel economy from extended open-loop operation
- Check engine light illuminated
Notes for Jeep Grand Cherokee
Grand Cherokee Pentastar 3.6L thermostat housing is prone to cracking; use updated Mopar part.