Powertrain, generic (extended)

P2CF1 Auxiliary Thermostat B Stuck Open

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P2CF1 covers a further auxiliary thermostat, labelled B, that stays open, so its loop loses its warm-up phase and runs below the intended temperature. Which loop B controls is the maker's choice: engines with split cooling may regulate head, block, oil or heater flow with separate thermostats, each needing its own temperature sensor for diagnosis. No manufacturer documents P2CF1 publicly, and the only published auxiliary thermostat case, Lotus bulletin 131-26007, concerns thermostat A (P2CEF). CARB demands that the OBD system detect a thermostat that keeps coolant from reaching its regulating temperature and, for gasoline vehicles from 2019–2021, one that lets it drop again; for multiple thermostats it asks for an approved plan. Fuel use and heater output suffer.

Likely causes

Valve held open by debris or a weak return spring
Worn wax element that no longer retracts fully
Temperature sensor of that loop reading too low and faking the fault

Symptoms

  • Engine warning lamp
  • Slow warm-up and higher fuel use in cold weather
  • Weaker cabin heating

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€120–800
Time1–4 h

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

Parts involved

aux thermostat bengine coolant loop

Related codes

P2CF2P2CEFP0128P026F

References: law.cornell.edu, en.wikipedia.org, static.nhtsa.gov.