Cooling system
P0128 — Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)
The coolant has not reached the thermostat's regulating temperature within the expected time. In most cases the thermostat is stuck open, so the engine runs cooler than designed.
Common causes
- Thermostat stuck open
- Faulty coolant temperature sensor
- Low coolant level
- Cooling fan running too much
- Wrong thermostat fitted after a repair
Symptoms
- Heater output weak in cold weather
- Temperature gauge reads low
- Higher fuel consumption
- Check engine light
How to check
- Check coolant level and condition
- Watch ECT as the engine warms: a thermostat should open at its rated temperature (about 80-95 deg C)
- Feel the upper radiator hose: it should stay cool until the thermostat opens
- Replace the thermostat if it opens too early; also check the sensor
Can you keep driving?
Usually safe to drive, but a cold engine wears faster and uses more fuel. Replace the thermostat soon.
Related codes
Frequently asked questions
- What does P0128 mean?
- The engine coolant is not warming up to the thermostat's opening temperature in the expected time.
- Can I drive with P0128?
- Yes in most cases, but the thermostat should be replaced to avoid extra wear and fuel use.
General reference information. It does not replace a diagnosis on your own vehicle. The same code can have different causes depending on make, model year and engine; check the manufacturer's documentation or ask a technician with a scan tool.