P0128
Coolant Thermostat Below Regulating Temperature
The engine has been running long enough to reach operating temperature, but coolant temperature stayed below the thermostat's regulating value. Almost always caused by a thermostat stuck open. Costs fuel economy and increases cold-engine emissions.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
What we see in the workshop
ON FORD ECOBOOST
Coolant below thermostat regulating temperature almost always means a stuck-open thermostat. Cheap to fix, but worth doing — it hurts fuel economy and, left alone, masks other cooling faults.
Common causes
Ordered by how often each turns out to be the cause.
- Thermostat stuck openCommon
- Coolant sensor reading lowLess likely
Parts involved
- Thermostat
- Coolant Temperature Sensor
P0128 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.