P0128: the thermostat is stuck open
The engine is taking longer than expected to reach normal operating temperature, usually because the thermostat is stuck open. It is not an emergency, but worth fixing.
Can you keep driving?
Yes. It is not an emergency, but it is worth fixing.
What the code means
The thermostat is meant to hold the coolant back until the engine is warm.
If it stays open, coolant circulates the whole time and the engine never reaches the right temperature.
Common causes
- A thermostat stuck in the open position
- A faulty temperature sensor
What to do now
- Watch the temperature gauge while driving. Does it never come up?
- Note whether the cabin heat is weaker than it used to be.
- If you notice this in autumn, deal with it before the cold arrives.
What people often get wrong
Dismissing it as a pure emissions code. In cold weather it is very noticeable in practice: poor heat, slow defrosting and higher fuel consumption every single day.
When the car must go to a workshop
This is usually a straightforward and relatively inexpensive repair once it has been confirmed.
Read next
This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.
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