Powertrain, generic (extended)

P25AF Coolant Temperature Control Module Requested MIL Illumination

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

P25AF is a messenger code. A separate coolant temperature control module, used on some vehicles with electrically managed cooling for engine, battery or power electronics, has stored an emission-relevant fault of its own and asks the engine module to turn on the check engine lamp. CARB's OBD II rule expects lamp requests from other powertrain units to be passed on reliably. No manufacturer publicly documents P25AF with this meaning; the nearest documented relatives are GM's brake module requests P25A2 and P25C9, held in the engine module as Type A without a separate lamp command. Volkswagen's 2014 Touareg Hybrid data book still used P25AF for an engine hood switch message fault on CAN. The actual cause has to be read out of the thermal module.

Likely causes

Emission-relevant fault stored in the coolant temperature control module
Faulty coolant valve, pump or sensor handled by that module
Thermal module software needing an update

Symptoms

  • Check engine lamp on
  • Possible temperature warnings or reduced heating

Repair estimate

DifficultyHardWorkshop job
Parts and labour€50–1,200
Time0.5–3 h

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

Parts involved

coolant temp control module

Related codes

P25A2P25C9P0700P2561

References: law.cornell.edu, gsi.ext.gm.com, static.nhtsa.gov.