P2508 sets when the engine control module detects a higher-than-expected voltage on the circuit that supplies it with switched/ignition-on power. This usually points to a problem in the external power feed path rather than the module itself.
A relay with welded contacts or incorrect coil resistance can feed excessive or unregulated voltage to the PCM ignition-power circuit, causing the input signal to read high.
The wire that carries ignition-switched power to the PCM may be rubbing against a battery-positive source, raising the monitored input above the expected threshold.
A failing voltage regulator or alternator can push system voltage above normal, which the PCM interprets as a high power-input signal.
High-resistance or intermittent connections in the power distribution path can create abnormal voltage readings at the PCM connector.
If the power feed, relay, charging system, and wiring all test normal, the PCM's internal voltage-sensing circuit may be misreading the input, but this is uncommon.
No. This code is most often caused by a relay, wiring short, or charging-system issue outside the PCM. Replacing the PCM before checking those items is usually unnecessary and expensive.
Yes. An overcharging alternator or failed voltage regulator can raise system voltage enough that the PCM sees its power input as abnormally high.
If the PCM does not see a stable, correct power input voltage, it may fail to power up properly or may shut down, leading to no-start, hard starting, or stalling.
Testing it first is reasonable because a stuck or incorrect relay is a common cause, but verify the relay specification and circuit condition rather than simply swapping parts at random.