OBD CodesWhat it means
The thermostat keeps coolant inside the engine until it is warm, then lets it flow to the radiator. These codes mean the engine is taking too long to warm up, or never reaches its normal temperature. The usual reason is a thermostat stuck open.
Codes in this group
| Code | Meaning |
|---|---|
| P0125 | Engine does not get warm enough for normal fuel control |
| P0126 | Engine does not reach a steady working temperature |
| P0128 | Engine stays below the thermostat temperature |
| P2181 | Cooling system not performing as expected |
Bank 1 is the side of the engine that has cylinder 1. Sensor 1 is before the catalytic converter and sensor 2 is after it. More terms explained.
What you may notice
- Temperature gauge stays low
- Heater blows only lukewarm air
- Higher fuel use, most of all in winter
Common causes
Most likely first:
- A thermostat stuck open
- Low coolant level
- A faulty coolant temperature sensor
- A radiator fan that runs all the time
What to check first
- Watch the temperature gauge on a 15 minute drive: it should rise to the middle and stay there
- Check the coolant level when the engine is cold
- A stuck thermostat is a cheap part, but leaving it wears the engine faster over time
Have a mechanic confirm. A fault code tells you which system reported a problem, not which part has failed. The same code can have several causes, and they differ between car models. Use this page to understand the code, then have a qualified mechanic test the car before any part is replaced.
After the repair
Once the cause is fixed, the code can be cleared with a scan tool, or the warning light goes out by itself after several trips without the fault. Clearing a code without fixing the cause only hides it for a while. More about clearing codes.
Related codes
Mass air flow (MAF) sensor (P0100), Manifold pressure (MAP) and barometric sensor (P0105), Intake air temperature sensor (P0110), Engine coolant temperature sensor (P0115), Throttle position sensor (P0120), Accelerator pedal sensor and throttle agreement (P2120), Electronic throttle actuator (P2100), Oxygen sensor before the catalytic converter (sensor 1) (P0130). All codes.
Written in November 2026. These are generic OBD-II codes, the same on all makes. Causes and repair steps differ between models.