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P codepowertrainMedium severity

P0652Sensor Reference Voltage "B" Circuit Low

The PCM's 5-volt sensor reference "B" circuit is reading lower than expected, usually because something on that shared supply is shorted to ground.

Definition

The PCM monitors each regulated 5-volt reference supply it produces, and P0652 sets when the "B" circuit voltage falls below the low threshold, indicating a short to ground or an overloaded supply. Because several sensors share this circuit, they all report near-zero signals at once, so this code is typically accompanied by multiple sensor circuit-low codes. Symptoms range from an illuminated MIL with default (substituted) sensor values to reduced engine power or a no-start if pedal or throttle sensors are on the affected bus.

Common causes

  • Sensor internally shorted to ground, pulling down the shared reference (most common)
  • Reference wire chafed against the engine, a bracket, or body metal and shorted to ground
  • Water or coolant intrusion in a sensor connector creating a path to ground
  • Harness damage from rodents or previous collision/repair work
  • Reference circuit pinched during recent service (intake, valve cover, or transmission work)
  • Failed internal PCM voltage regulator (rare)

Common misdiagnoses

  • Replacing each sensor that logged a circuit-low code instead of finding the one shorted device
  • PCM replaced without unloading the reference circuit first
  • Ground straps or battery replaced for what is a localized short
  • Throttle body replaced to address the reduced-power symptom

Troubleshooting steps

  1. 1.Map the "B" reference sensor group

    Use the vehicle wiring diagram to list every sensor fed by reference "B", since disconnect testing must cover the whole group.

  2. 2.Confirm the low reading

    Key on, backprobe the reference at any affected sensor; a reading well below 4.9 volts (often near 0) confirms the supply is loaded down.

  3. 3.Unplug sensors one at a time

    Watch the reference voltage as each sensor on the circuit is disconnected; recovery to 5 volts identifies the shorted sensor, which should be replaced.

  4. 4.Test the harness to ground

    If no single sensor restores the supply, disconnect the PCM and measure resistance from the reference wire to ground; anything other than open (OL) indicates a harness short to locate and repair.

  5. 5.Clear codes and verify under wiggle test

    After the repair, clear codes, flex the harness sections near the repair while watching live reference-fed sensor data, and confirm the code does not return.

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