P2505 indicates the engine control module (ECM/PCM) has detected an abnormal voltage condition on its main power input circuit. The module continuously monitors its supply voltage; if voltage falls below, exceeds, or becomes unstable relative to calibrated thresholds, this code is stored.
A battery that cannot maintain sufficient voltage under load or during cranking will cause the PCM to detect low system voltage.
An alternator that over-charges, under-charges, or produces excessive AC ripple can trigger the code.
Corroded, loose, or damaged wiring, terminals, or fuse-block contacts between the battery/ignition switch and the PCM create voltage drop.
Worn ignition switch contacts or a failing relay can supply intermittent or low voltage to the PCM power input.
Internal voltage-regulator or monitoring-circuit failure inside the PCM is possible but rare compared to external wiring and battery issues.
Not usually. The majority of P2505 occurrences are caused by battery, charging system, or wiring faults upstream of the PCM. PCM replacement should only be considered after the power and ground circuits have been proven intact.
Yes. An alternator that produces low voltage, high voltage, or excessive AC ripple will cause the PCM to detect an abnormal power input signal.
When system voltage is unstable, multiple modules and sensors can set plausibility or communication codes as a secondary effect of the same power-supply issue.
The vehicle may stall without warning or fail to restart. The root cause should be diagnosed and repaired before relying on the vehicle for normal operation.