Powertrain, generic (extended)

P2CEF Auxiliary Thermostat A Stuck Open

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P2CEF concerns a second thermostat that regulates its own part of the cooling system; stuck open means coolant passes through it before the intended temperature, so that circuit warms too slowly or stays too cool. Lotus documents the code in bulletin 131-26007 for the four-cylinder Emira, where thermostat A is the medium-temperature thermostat: the MIL may light and the driver may see a warning about low coolant temperature; the coolant is filtered for particles and the cold thermostat checked for a recessed piston and free travel in hot water. Mercedes-AMG lists P2CEF00 on M139 engines (CLA 45/GLA 45 facelift) with its own wording, an electrical fault of the thermostat, and blames debris, fixed by a revised thermostat. CARB requires monitoring of multi-thermostat systems.

Likely causes

Debris in the coolant holding the thermostat valve off its seat
Worn or leaking wax element, piston no longer returns
Earlier thermostat version superseded by the maker (Mercedes M139)

Symptoms

  • Engine warning lamp
  • Low coolant temperature warning or slow warm-up
  • Weaker cabin heating in cold weather

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€150–900
Time1–4.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

aux thermostat aengine coolant loop

Related codes

P2CF0P2CF1P0128P026F

References: static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov, law.cornell.edu.