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P codepowertrainMedium severity

P20D5Exhaust Aftertreatment Fuel Injector "B" Control Circuit High

The control circuit for the second ("B") exhaust aftertreatment fuel injector is reading higher voltage than expected, usually from a short to battery voltage or a stuck-off driver.

Definition

The ECM drives the "B" hydrocarbon doser injector to inject fuel into the exhaust for active DPF regeneration and continuously monitors the driver circuit's feedback voltage. P20D5 sets when that circuit reads above the expected range, indicating a short to voltage, excessive circuit resistance, or a driver that cannot pull the circuit low. The ECM suspends active regeneration and turns on the MIL; prolonged operation allows soot loading, follow-on DPF restriction codes, and eventually a power derate.

Common causes

  • Control wire shorted to battery or ignition voltage in a chafed harness section
  • Corrosion or high resistance in the injector connector raising circuit voltage
  • Open injector solenoid coil
  • Water intrusion at the connector bridging circuits
  • Failed ECM driver stage

Common misdiagnoses

  • DPF cleaned or replaced for the soot loading that follows the failed regens
  • DOC replaced for low regeneration temperatures
  • Doser injector replaced without finding the wiring short
  • EGT sensors swapped for a purely electrical circuit code

Troubleshooting steps

  1. 1.Back-probe the control circuit key-on

    Constant battery voltage on the control wire with the injector commanded off confirms a short to voltage or an open driver path.

  2. 2.Check injector coil continuity

    An open coil (infinite resistance) will read as circuit high on many strategies; compare against the OEM resistance spec.

  3. 3.Inspect the connector for corrosion and moisture

    Green terminals, water staining, or spread terminals create high resistance; verify terminal drag with a test pin.

  4. 4.Wiggle-test the harness while monitoring the circuit

    Watch live voltage or the code status while flexing the loom near heat shields and tie-down points to catch intermittent shorts.

  5. 5.Command the injector and verify switching

    Using a bidirectional scan tool, confirm the circuit toggles between supply voltage and near zero; a circuit stuck high with good wiring indicates the ECM driver.

  6. 6.Confirm the repair with a forced regeneration

    Verify normal dosing behavior and exhaust temperature rise with no code reset.

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