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P codepowertrainMedium severity

P0698Sensor Reference Voltage "C" Circuit Low

The 5-volt sensor reference "C" circuit is reading lower than expected, usually because a sensor or wiring short to ground is dragging the shared supply down.

Definition

The PCM monitors each of its regulated 5-volt reference buses, and P0698 sets when the reference "C" bus voltage drops below its low threshold. A short to ground anywhere on the bus — inside one of the sensors it feeds or in the harness — pulls the supply down for every sensor sharing it, so multiple sensor low-voltage codes typically set together. Expect an illuminated MIL and frequently reduced-power or limp-home operation until the short is found.

Common causes

  • Internally shorted sensor on the reference C bus (pressure, position, or pedal sensor depending on make)
  • Reference wire chafed through and shorted to ground
  • Water intrusion shorting a shared connector
  • Harness pinched at a bracket or after recent repair work
  • PCM internal regulator fault

Common misdiagnoses

  • Replacing each sensor that set a companion low-signal code
  • PCM replaced before isolating the shorted branch of the bus
  • Battery or charging system suspected for the low-voltage readings

Troubleshooting steps

  1. 1.Record companion codes to map the bus

    Sensors reporting simultaneous low-signal faults are the ones sharing reference C — verify the list against the wiring diagram.

  2. 2.Measure the bus voltage key-on

    Backprobe the reference terminal at any shared sensor; a reading well below 4.9 volts confirms the bus is loaded down.

  3. 3.Disconnect shared sensors one at a time

    When the voltage snaps back to about 5.0 volts after unplugging a sensor, that sensor is internally shorted.

  4. 4.Isolate the harness if sensors test good

    Disconnect the PCM and all bus sensors, then check the reference wire for continuity to ground to find a chafe or pinch point.

  5. 5.Confirm the PCM output

    With the bus unloaded, the PCM pin should output 4.9–5.1 volts; a low output with no external load indicates the internal regulator.

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