P25C1

Code

Generic P — Powertrain

Alternative Fuel Disable Signal Circuit

Brand: Generic

Type: P — Powertrain

Views: UK: 29 EN: 32 RU: 23

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Page language: EN

Causes

Open or shorted wiring in the alternative fuel disable signal circuit
Corroded, damaged or loose connector/pins at the sensor/module/ECM
Faulty alternative-fuel disable switch or module (LPG/CNG ECU)
Blown fuse or faulty relay that feeds the disable circuit
Poor ECM power or ground reference
Intermittent contact from vibration or corrosion

Symptoms

MIL (Malfunction Indicator Lamp) illuminated or stored DTC
Vehicle may fail to switch to alternative fuel or may revert to gasoline
Reduced drivability or limp-home behavior when system defaults
Intermittent loss of alternative-fuel functionality
Possible inability to start on alternative fuel in some systems

What to check

Retrieve DTC and freeze-frame data with a scan tool; note conditions when fault set
Visually inspect wiring, connectors, and pins for damage, corrosion, or water ingress
Check relevant fuses and relays that supply the alternative-fuel disable circuit
Backprobe the disable signal at the module and at the ECM; monitor voltage with key ON and during activation
Perform continuity and resistance checks between the module switch and ECM
Wiggle harness while monitoring signal to locate intermittent faults

Signal parameters

Typical signal types: switched ground, switched battery (12 V) or low-voltage logic (0–5 V) depending on vehicle
Common behavior: near 0 V (ground) or logic LOW when disabled, ~5 V or ~12 V (logic HIGH) when enabled — consult OEM wiring diagram
Expected current draw is usually very low (signal level only); load circuits (relays) will draw higher current
Open-circuit or voltage out of expected range (above expected max or below expected min) triggers the fault
Resistance from signal wire to ground when circuit open should be very high (kΩ range); short to ground will show near 0 Ω

Diagnostic algorithm

Confirm the code with a scan tool; record freeze-frame and any related P‑codes (fuel system, communication, powertrain).
Perform a visual inspection of wiring and connectors for the alternative-fuel disable circuit (follow harness from alternative-fuel module to ECM).
Check fuses and relays for the alternative-fuel system; replace any blown fuses and retest.
With key ON (engine off), backprobe the disable signal at the module and at the ECM. Verify voltage state matches expected OEM values for enabled/disabled positions.
Operate the disable switch (or enable/disable system) while monitoring the signal. Note if voltage changes correctly and consistently.
If voltage is incorrect or intermittent, check continuity of the wire between module and ECM; repair broken wires or terminals.
Inspect and test connector pins for corrosion or damage; clean or repair as needed.
Check power and ground circuits to the alternative-fuel module and ECM. Repair poor grounds or power feeds.
If wiring and module tests good but fault remains, swap in a known-good module (if available) or perform ECM bench tests per OEM procedure.
After repairs, clear codes and perform a road test or functional test to confirm the fault does not return.

Likely causes

Damaged harness or connector between alternative-fuel module and ECM
Failed disable switch/module (stuck open or shorted)
Blown fuse or bad relay supplying the circuit
Corroded/loose pin at connector causing intermittent/open circuit
ECM input driver failure (least likely after wiring checks)

Fault status

⚠️ Status

Stored/Active — ECM detected invalid or out-of-range voltage on the Alternative Fuel Disable Signal circuit. MIL may be illuminated and alternative-fuel operation may be disabled.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0 - 3.0 hours