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P codepowertrainMedium severity

P0407Exhaust Gas Recirculation Sensor "B" Circuit Low

The signal voltage from the secondary EGR sensor (Sensor B) is stuck below its normal operating range.

Definition

On engines with more than one EGR-related sensor, the PCM monitors the Sensor "B" circuit — typically a second EGR valve position sensor or an exhaust pressure sensor used for EGR flow monitoring — and expects its voltage to stay within a plausible window. P0407 sets when the signal drops below the minimum threshold (commonly under about 0.15V), indicating a short to ground, an open reference feed, or a dead sensor. Driveability impact is usually mild, but the MIL illuminates, the PCM may disable or limit EGR operation, and the vehicle can fail an emissions test; on diesels, restricted EGR control can also affect NOx strategy.

Common causes

  • EGR sensor B signal wire shorted to ground
  • Failed EGR position or pressure sensor
  • Open or shorted 5V reference supply to the sensor
  • Corroded or damaged connector terminals
  • Harness chafed through near the EGR valve (high-heat area)
  • Faulty PCM (rare)

Common misdiagnoses

  • Complete EGR valve replaced when only the sensor circuit wiring was faulty
  • Sensor replaced when the shared 5V reference was shorted by a different sensor
  • EGR passages cleaned for what is purely an electrical circuit code
  • PCM replaced without checking the reference voltage

Troubleshooting steps

  1. 1.Read the sensor voltage on the scan tool

    Key on, engine off, confirm the Sensor B parameter is pegged low (near 0V) rather than fluctuating, which confirms a hard circuit fault.

  2. 2.Check the 5V reference at the sensor

    Backprobe the reference wire at the sensor connector; missing 5V means an open feed or a reference short elsewhere on the shared circuit.

  3. 3.Test for a signal short to ground

    Disconnect the sensor and PCM connectors and check that the signal wire shows no continuity to chassis ground.

  4. 4.Verify sensor response

    With the sensor connected, actuate the EGR valve with a bidirectional scan tool (or apply pressure to a pressure-type sensor) and confirm the voltage sweeps smoothly through its range.

  5. 5.Inspect the connector and harness

    Look for heat-hardened insulation, chafe points near the exhaust, and corroded terminals in the sensor connector.

  6. 6.Clear the code and retest

    After repair, run the EGR monitor or drive cycle and confirm the code does not reset and the readiness monitor completes.

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