Powertrain, generic (extended)

P25C1 Alternative Fuel Disable Signal Circuit

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

P25C1 relates to the incoming side of the alternative fuel disable function: a signal line on which the engine controller reads whether gas operation is blocked, either as feedback from the gas system or as a request from it, for example when a CNG or LPG controller detects low tank pressure or its own fault. The code says the signal is missing or unusable without naming a direction. In GM's bi-fuel Silverado and Sierra the system changes back to gasoline by itself when the gas tank runs empty and stops toggling after 20 transitions in one key cycle, which shows how much the engine controller depends on knowing the gas system's state. No manufacturer documents P25C1 publicly. P25C2 and P25C3 are the high and low variants; P25BE to P25C0 cover the control output.

Likely causes

Open signal wire from the gas system
Gas controller without power or ground, so no signal is sent
Corroded underbody connector of the gas system

Symptoms

  • Gas mode unavailable or drops out while driving
  • 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 modulefeedback sensorwiring harness

Related codes

P25C2P25C3P25BEP25A4

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