Thermostat
P0128
Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
The engine is taking too long to reach its normal temperature. The thermostat is most likely stuck open.
Fine to drive
Can I keep driving? Yes, in most cases. It won't hurt the car in the short term, but get it fixed. A check engine light that's already on can't warn you about a new problem. It's safe to drive. The engine runs cold, which costs fuel, wears the oil faster and gives you a weak heater.
Common causes
- A thermostat stuck open (by far the most common)
- Low coolant
- A coolant temperature sensor reading low
- Very cold weather on short trips
Listed roughly from most to least likely. The same code can have different causes on different cars.
What to do
- With the engine cold, check the coolant level.
- Notice whether the heater takes a long time to blow warm. That points to the thermostat.
- Replace the thermostat. It's usually an inexpensive job.
General guidance. Your owner's manual wins. If a red warning light is on or the car drives differently, stop safely and get help.
Track this automatically in Tagwell Auto
Tagwell Auto keeps your car's service dates and mileage, so when a code shows up you know what was done last. With a Bluetooth reader for the car's diagnostic port (called OBD-II), the phone app can read the code itself.