P0128Low severityPowertrainDIY: Moderate
P0128: Coolant Temperature Below Thermostat Regulating Temperature
The engine takes too long to warm up. The fix is almost always a stuck-open thermostat, but a faulty coolant temp sensor or cold-climate driving can also trigger it.
Can I keep driving?
Yes. It is not urgent, but an engine that runs cold for months wears faster and uses more fuel.
Symptoms you might notice
- Temperature gauge takes a long time to reach its normal spot, or never gets there
- Heater blows lukewarm air in cold weather
- Slightly worse fuel economy
What to check first
- With the engine cold, check the coolant level and top it off with the right coolant if it is low.
- Watch the temperature gauge on a normal drive. If it climbs slowly or sits low, a thermostat stuck open is the usual cause.
- If the gauge looks normal, compare the sensor reading on a scan tool with the temperature of the upper radiator hose to check the sensor.
Most common causes
- Stuck-open thermostat
- Faulty engine coolant temperature (ECT) sensor
- Low coolant level
Check it against your exact vehicle
A code means the same thing on every car, but where the parts are and what to check first depends on your year, make, model and engine. Tell CaliperAI what you are seeing and it will help narrow down the likely causes for your vehicle.
Not sure this is your problem? Browse common car problems by symptom.
General information about this code, not a diagnosis of your specific vehicle. Safety-critical work such as brakes, fuel and airbags is best left to a qualified mechanic.