What does code P0128 mean on a Ram and is it safe to drive?
P0128: Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature). Here's what it means on your Ram, what usually causes it, what repairs typically cost, and how safe it is to keep driving.
What this code means
The engine is not reaching normal operating temperature in the expected time. Usually a stuck-open thermostat.
Common causes for your Ram
- Thermostat stuck open
- Coolant temperature sensor fault
- Low coolant level
Symptoms
- Check Engine Light
- Heater may blow cold
- Poor fuel economy
- Temperature gauge stays low
Typical fixes
- Replace thermostat
- Replace coolant temperature sensor if faulty
- Refill coolant and check for leaks
Repair cost range
For a Ram, repairs for P0128 often fall between $100 and $350, depending on the cause and whether you use OEM or aftermarket parts. Labor rates vary by location.
Known issues on Ram
On Ram 1500 with the Pentastar 3.6L, P0128 diagnosis is the same as Dodge—a stuck-open thermostat. On Ram 2500 with the 6.7L Cummins diesel, P0128 may indicate a stuck-open thermostat or a thermostat housing coolant bypass valve failure, which is a known Cummins issue.
Most commonly affected models