P0128Severity: Low
P0128 – Coolant Thermostat Temperature Below Regulating
The engine does not reach operating temperature in the expected time.
What it means
The coolant does not reach its regulating temperature within the expected time. It is almost always caused by a thermostat stuck open.
Symptoms
- Engine warms up slowly
- Heater is slow to warm
- Slightly higher fuel use
Common causes
- Thermostat stuck open
- Faulty coolant temp sensor
- Low coolant level
- Cooling fan running constantly
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Frequently asked questions
What does trouble code P0128 mean?
P0128 – Coolant Thermostat Temperature Below Regulating. The coolant does not reach its regulating temperature within the expected time. It is almost always caused by a thermostat stuck open.
Can I drive with a P0128 code?
It's usually not urgent, but finding and fixing the cause matters for emissions and fuel economy.