P2CEF
Auxiliary Thermostat A Stuck Open
The auxiliary thermostat A is stuck open. Coolant flow bypasses the thermal regulation, leading to slow warm-up and reduced engine efficiency.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- Thermostat element failedCommon
- Wax pellet aged - element non-responsivePossible
- Carbon or debris buildup preventing free movementLess likely
Parts involved
- Aux Thermostat A
- Engine Coolant Loop
P2CEF 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.