Powertrain, generic (extended)

P25C3 Alternative Fuel Disable Signal Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P25C3 is logged when the alternative fuel disable signal reaching the engine controller is too low to be valid. A short to ground on the way from the gas system, a gas controller that has lost its supply, or a damaged output stage in that controller can each pull the line down. On a gas installation that announces a block by grounding the line, this would also be indistinguishable from a genuine stop request, so the engine controller has to treat gas operation as unavailable. GM's 2016 bi-fuel trucks illustrate the fallback: they start on gasoline and only change over once the conditions for gas operation have been met, and a failed changeover is shown with the fuel indicator. No manufacturer documents P25C3 publicly. The general signal fault is P25C1, the opposite case P25C2.

Likely causes

Signal wire shorted to ground
Gas controller without supply voltage
Damaged output stage in the gas controller

Symptoms

  • No change-over to gas operation
  • Fuel mode indicator flashes or shows a fault
  • 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

P25C1P25C2P25C0

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