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P codepowertrainMedium severity

P0408Exhaust Gas Recirculation Sensor "B" Circuit High

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

Definition

The PCM monitors the EGR Sensor "B" circuit — usually a second EGR position sensor or an EGR-related pressure sensor — and sets P0408 when the signal voltage exceeds the maximum plausible threshold (commonly above about 4.8-4.9V). A signal pegged high typically indicates an open ground circuit, a signal wire shorted to reference or battery voltage, or an internally failed sensor. The MIL illuminates and EGR control may be disabled or limited, which can cause light spark knock on gasoline engines, elevated NOx, and an emissions-test failure, though day-to-day driveability is usually only mildly affected.

Common causes

  • Open sensor ground (return) circuit
  • Signal wire shorted to the 5V reference or battery voltage
  • Failed EGR position or pressure sensor
  • Corroded or backed-out connector terminals
  • Heat-damaged harness near the EGR valve or exhaust
  • Faulty PCM (rare)

Common misdiagnoses

  • Complete EGR valve assembly replaced for an open ground wire
  • EGR passages decarboned for a purely electrical fault
  • Sensor replaced twice when the harness short remained
  • PCM replaced without load-testing the ground circuit

Troubleshooting steps

  1. 1.Confirm the pegged-high signal

    Key on, engine off, verify the Sensor B voltage reads at or near reference voltage (4.8V+) and does not change when the valve is actuated.

  2. 2.Unplug the sensor and watch the scan tool

    If the voltage stays high with the sensor disconnected, the signal wire is shorted to voltage in the harness; if it drops to 0V, suspect the sensor or its ground.

  3. 3.Load-test the sensor ground

    Measure voltage drop between the sensor ground terminal and battery negative with the circuit loaded; more than 0.1V indicates a poor or open return.

  4. 4.Check for wire-to-wire shorts

    With connectors unplugged at both ends, verify no continuity between the signal wire and the 5V reference or any power feed in the same harness section.

  5. 5.Sweep-test the sensor

    Actuate the EGR valve through its range with a bidirectional tool and watch for a smooth voltage sweep without dropouts or spikes before condemning the sensor.

  6. 6.Repair, clear, and run the monitor

    After the repair, complete an EGR monitor drive cycle and confirm the code stays cleared with the monitor reporting ready.

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