P0128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
The engine is taking too long to warm up, or never reaches normal temperature.
Common causes
- Thermostat stuck open
- Faulty coolant temperature sensor
- Low coolant level
Symptoms
- Heater blows lukewarm
- Temperature gauge reads low
- Worse fuel economy
Can I drive with it?
Usually safe to drive, but check the coolant level first – and in cold weather a weak heater can mean a foggy windshield.
Tip: A thermostat that sticks open is the most common cause and is usually an inexpensive part.
Which scanner reads it?
Any OBD2 scanner reads it. Live coolant-temperature data shows whether the engine is really running cold.
Budget tools like the ANCEL AD310 read it; tools with live data and graphing – like the BlueDriver or OBDLink MX+ – help you find the cause. Not sure which fits you? Try the Find My Scanner quiz.
In the data
How common is it? At New York’s emissions inspections in 2020–21, P0128 was the #16 code found on cars that failed – 1.3% of all the codes found.
Of the vehicle complaints owners filed with NHTSA from 2015 to 2025, 75 quote P0128 – most often from Chevrolet, Jeep, GMC owners. Complaints lean toward serious problems, so treat this as a sign of how often the code turns up in real reports, not how often cars get it.
Both sources are compared on the Trends page.
General information to help you understand a code – not a diagnosis. Your car’s service information or a qualified mechanic has the final word. See how we research.