Code P0128 (Coolant Thermostat Below Regulating Temperature) on the 6.7L Cummins means the engine is not reaching or holding its proper operating temperature. This guide covers what triggers P0128, how to diagnose it, and the parts that fix it.
What P0128 means
P0128 sets when the coolant does not warm up as expected in the time/temperature the ECM models - almost always a thermostat stuck open (or partially open).
Common causes
- Thermostat stuck open
- A wrong or failed thermostat
- A faulty coolant temperature sensor
- Extended idling in very cold weather
Symptoms you'll notice
- Long warm-up and an engine that runs cool
- Poor heater output
- Reduced fuel economy
- A check-engine light
How to diagnose it
Confirm the coolant warms to and holds operating temperature. A thermostat stuck open lets it run cool; verify the coolant temp sensor reads correctly before replacing parts.
The fix
Replace a stuck-open thermostat with the correct unit and refill with proper coolant, bleeding the system. If the thermostat is fine, check the coolant temp sensor. Correct operating temperature restores economy and heat.
Frequently asked questions
Can I drive with P0128?
Yes, but the engine runs cool, hurting economy and heat. Replace the thermostat promptly.
Is it the thermostat or the sensor?
A stuck-open thermostat is the most common cause; verify the coolant temp sensor if a new thermostat does not fix it.
Need the part?
We stock the 6.7L Cummins Thermostat - New Direct-Fit Diesel Replacement for your 6.7L Cummins and ship it direct. Shop it →