Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)
The engine coolant did not reach the required temperature within the expected time, usually indicating the thermostat stays open too long.
Common causes
- Stuck-open thermostat
- Faulty coolant temperature sensor
- Low coolant level
Possible symptoms
- Check Engine Light on
- Low temperature gauge reading
- Reduced fuel economy
- Weak cabin heat
Can I keep driving?
You can usually keep driving: this is a low-severity fault. It's still worth having it checked soon so it doesn't get worse or cause an emissions-test failure.
Code breakdown
Frequently asked questions
What does code P0128 mean?
P0128 stands for "Coolant Thermostat (Coolant Temp Below Thermostat Regulating Temperature)". The engine coolant did not reach the required temperature within the expected time, usually indicating the thermostat stays open too long.
Can I keep driving with code P0128?
You can usually keep driving: this is a low-severity fault. It's still worth having it checked soon so it doesn't get worse or cause an emissions-test failure.
How do I diagnose and fix code P0128?
Diagnosis starts with the most common causes, checked in this order: Stuck-open thermostat; Faulty coolant temperature sensor; Low coolant level. Reading live data with an OBD-II tool helps confirm the fault before replacing any parts.
Related codes
- P0100 — Mass or Volume Air Flow Circuit
- P0101 — Mass or Volume Air Flow Circuit Range/Performance
- P0102 — Mass or Volume Air Flow Circuit Low Input
- P0103 — Mass or Volume Air Flow Circuit High Input
- P0104 — Mass or Volume Air Flow Circuit Intermittent
- P0105 — Manifold Absolute Pressure/Barometric Pressure Circuit