P0128 — Coolant Thermostat Below Regulating Temperature
Seen on: Heavy Truck
Coolant Thermostat Below Regulating Temperature. Common causes: Thermostat stuck open or missing.
Your code rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Thermostat stuck open or missing
- Failed thermostat
- thermostat housing gasket bypass
First moves (roadside)
- Monitor warmup time
- engine should reach operating temp within 5 min
- replace thermostat
FAQ
Coolant Thermostat Below Regulating Temperature. Common causes: Thermostat stuck open or missing.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Thermostat stuck open or missing. Other possibilities include Failed thermostat; thermostat housing gasket bypass.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack and more). Not a substitute for a scan-tool diagnosis on your specific truck.