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P255BPTO Speed Selector Sensor/Switch 2 Circuit Range/Performance

The signal from PTO speed selector sensor/switch 2 is out of its plausible range or inconsistent with the commanded PTO operation.

Definition

The PCM rationality-checks the second PTO speed selector input against its calibrated range and, on some applications, against the first selector input. P255B sets when input 2 reads a value that is electrically valid but implausible — a voltage in a dead band, a state disagreeing with selector 1, or a reading that never changes. PTO speed control may be disabled, limited, or default to base idle, sometimes with a MIL; the truck drives normally otherwise.

Common causes

  • Worn or faulty second speed selector switch/potentiometer
  • High resistance from corroded terminals in the selector or upfitter connector
  • Incorrect upfitter wiring crossing or loading the selector inputs
  • Chafed harness partially shorting the signal
  • PCM PTO configuration mismatched to the installed selector type

Common misdiagnoses

  • Replacing the PCM
  • Replacing the PTO unit
  • Replacing selector switch 1 instead of input 2's switch or wiring
  • Accelerator pedal sensor replacement

Troubleshooting steps

  1. 1.Verify configuration and wiring assignments

    Confirm the PCM PTO parameters and the upfitter pinout match the installed equipment — swapped selector 1/2 wires are a classic install error that sets rationality codes.

  2. 2.Sweep input 2 on live data

    Operate the second selector through its full range and confirm the PID changes smoothly and lands in valid windows at each position.

  3. 3.Measure the selector output directly

    Backprobe the signal and compare against spec (a smooth 0.5-4.5V sweep for potentiometer types, or discrete resistance steps for multi-position switches).

  4. 4.Check for circuit loading

    Inspect for aftermarket equipment or moisture paths adding resistance to the circuit, which pulls the signal into implausible mid-range values.

  5. 5.Replace the selector if the circuit is clean

    With wiring and configuration verified, fit a new selector, clear the code, and confirm PTO RPM control follows each selected speed.

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