DTC codes / P25xx · Vehicle speed, idle control and auxiliary inputs / P251B
P251B indicates that the powertrain control module (PCM) has detected a low-voltage condition on the power take-off (PTO) enable switch circuit, which is used to engage auxiliary equipment driven by the engine. The PCM expects to see a specific voltage on the switch signal circuit when the switch is in the off or on position, and a low reading tells the module that the circuit is shorted to ground or the switch is stuck closed.
Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.
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