P0128 Code: Coolant Thermostat (Coolant Temperature Below Regulating Temperature)
🟠 Moderate severity · drivable
Safe to drive, but the engine running too cool hurts fuel economy and heater performance, so replace the thermostat soon.
What P0128 means
The engine is taking too long to reach (or isn't maintaining) its normal operating temperature. Usually this means the thermostat is stuck open, letting coolant circulate through the radiator constantly so the engine never fully warms up.
Symptoms
- Check engine light is on
- Engine temperature gauge reads low
- Heater blows lukewarm air
- Reduced fuel economy
Common causes
- most common: Thermostat stuck open
- common: Low coolant level
- less common: Faulty engine coolant temperature (ECT) sensor
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Check and top up the coolant if it's low | Yes | $10–30 |
| Replace the thermostat | Yes | $20–80 |
| Replace a faulty coolant temperature sensor | Yes | $30–100 |
Frequently asked questions
Can I drive with a P0128 code?
Yes. The engine just runs cooler than normal. It's not an emergency, but fix it soon for better fuel economy and heat.
What is the most common cause of P0128?
A thermostat stuck open. It's a common, inexpensive part to replace.
How much does it cost to fix a P0128?
Usually $20–$80 for the thermostat itself, plus about an hour of labor if you have a shop do it.
Related codes
P0117
P0117 Code: Engine Coolant Temperature Circuit Low Input
P0118P0118 Code: Engine Coolant Temperature Circuit High Input
P0125P0125 Code: Insufficient Coolant Temperature for Closed Loop Fuel Control
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.