P0126
Insufficient Coolant Temperature for Stable Operation
Coolant temperature stays below the value expected for stable engine operation. Almost always points to a thermostat stuck open or a sensor reading lower than the actual engine temperature.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- Thermostat stuck openCommon
Parts involved
- Thermostat
- Coolant Temperature Sensor
P0126 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.