P25C3

Code

Generic P — Powertrain

Alternative Fuel Disable Signal Circuit Low

Brand: Generic

Type: P — Powertrain

Views: UK: 22 EN: 29 RU: 18

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Wiring shorted to chassis ground on the alternative-fuel disable circuit
Open or high-resistance connector or corroded terminal at the alternative-fuel module or ECU
Failed alternative-fuel module or disable switch (internal short)
Blown fuse, faulty relay or low supply voltage feeding the disable circuit
Damaged harness (chafing, rodent damage) causing intermittent/low-voltage condition
Control module input driver fault or software anomaly

Check Engine light (MIL) illuminated
Alternative-fuel mode disabled or cannot be selected; vehicle defaults to gasoline/petrol
Possible reduced fuel economy or inability to run on alternative fuel
Inconsistent behavior when switching fuels; may be intermittent depending on harness movement

Read and record freeze frame and all stored codes with a scan tool
Visually inspect wiring, connectors and harnesses between ECU and alternative-fuel controller for damage, corrosion, or unsecured routing
Check related fuses and relays for the alternative-fuel system
Backprobe the disable signal circuit at the connector with key ON (engine off) and observe voltage
Wiggle test harness and connectors while monitoring signal for intermittent drops
Unplug the alternative-fuel controller (if service manual allows) and re-check the line to see if the voltage returns to expected level

Expected: logic-high or pulled-up voltage when disable is inactive — commonly ~5 V (for logic circuits) or ~12 V (for vehicle-level switched circuits) depending on design
Fault reading: low voltage / near 0 V indicates short-to-ground or active low condition
Typical thresholds (generic): low 4.5 V for 5 V systems or >10 V for 12 V systems — consult vehicle-specific manual for exact values

Connect a scan tool, confirm P25C3 and capture freeze frame data (engine state, battery voltage, temperatures).
Perform a visual inspection of the alternative-fuel controller, ECU connectors and wiring; repair obvious damage.
Check fuses/relays for the alternative-fuel system and replace if blown. Verify supply and ground at the module.
With key ON (engine off), backprobe the disable signal at the module and ECU connectors and measure voltage to ground. Compare to expected (see service manual).
If voltage is low, disconnect the alternative-fuel module connector; if the line returns to high, suspect the module (internal short). If it stays low, suspect a short in the harness or ECU input.
Use an ohmmeter to check continuity from the disable pin to chassis ground to locate a short. Also check resistance to supply/pull-up. Repair wiring or connector faults.
If harness and connectors are good, test or replace the alternative-fuel controller per manufacturer procedure. If ECU input is suspected, consult manufacturer procedures before replacing ECU.
After repairs, clear codes and perform a functional test: verify the disable input voltage is within spec and confirm alternative-fuel operation under the same conditions shown in freeze frame.
If issue is intermittent, perform wiggle tests with a partner monitoring live data and consider wiring harness replacement if fault location is not repairable.

Short to ground in the disable signal wire between ECU and alternative-fuel module
Corroded/loose connector at the alternative-fuel controller or ECU pin
Failed alternative-fuel controller pulling the line low
Blown fuse or fault in the module power/ground that changes the expected signal level