Powertrain, generic (extended)

P25BF Alternative Fuel Disable Control Circuit High

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P25BF flags a voltage on the alternative fuel disable output that is too high for the state the engine controller is commanding. On a low-side driver this usually means a short to battery or ignition voltage, for instance where a gas kit harness has been routed along a hot feed or tapped into the wrong wire. Depending on design, a constantly high line may be read by the gas system as a permanent block, or the controller may switch the driver off to protect itself; either way the command it sends is no longer trustworthy. GM's 2016 Silverado and Sierra bi-fuel trucks show what the driver sees when gas operation is refused: the fuel indicator flashes and the truck stays on gasoline. No manufacturer documents P25BF publicly. Note that SAE lists High before Low in this group.

Likely causes

Control wire shorted to battery or ignition voltage
Retrofit gas kit tapped into the wrong circuit
Faulty input stage in the gas controller backfeeding voltage

Symptoms

  • Gas operation blocked, vehicle runs on gasoline only
  • Fuel mode indicator flashes
  • Check engine lamp

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–600
Time0.5–3 h

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

Parts involved

alt fuel control modulewiring harness

Related codes

P25BEP25C0P25C2

References: gmc.com, law.cornell.edu.