Powertrain
Definition only
Lights the check engine lamp
P2CEF: Auxiliary Thermostat A Stuck Open
Standardized SAE J2012 code · P2Cxx codes
What does P2CEF mean?
The auxiliary thermostat A is stuck open. Coolant flow bypasses the thermal regulation, leading to slow warm-up and reduced engine efficiency.
The code tells you which monitor failed, not which part is broken. Test before replacing parts.
Common causes
- High Thermostat element failed
- Medium Wax pellet aged - element non-responsive
- Low Carbon or debris buildup preventing free movement
Likelihood is a general estimate across vehicles; your make and model may differ.
Add your car to see reports of P2CEF on the same make and model first.
P2CEF in the community
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