P0128 RAM 1500
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 RAM 1500 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 RAM 1500.
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 RAM 1500.
Common Causes on RAM 1500
- Thermostat stuck open on 5.7L Hemi V8 or 3.6L Pentastar V6
- Thermostat housing gasket leak causing slow coolant loss
- Coolant temperature sensor providing inaccurate readings
- Low coolant from weeping water pump on high-mileage Hemi engines
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 RAM 1500
Ram 1500 Hemi thermostat is located at the front of the engine; easy to reach from above.