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P codepowertrainMedium severity

P20D4Exhaust Aftertreatment Fuel Injector "B" Control Circuit Low

The control circuit for the second ("B") exhaust aftertreatment fuel injector is reading lower voltage than expected, typically from a short to ground or an open circuit.

Definition

On diesels with active DPF regeneration, the ECM pulses a dedicated hydrocarbon doser injector to spray fuel into the exhaust stream ahead of the oxidation catalyst, raising temperature to burn accumulated soot; this code covers the "B" (second) injector on systems that use more than one. P20D4 sets when the ECM's driver circuit for that injector sees voltage below the expected range while monitoring the circuit. With the code active, the ECM disables active regeneration, illuminates the MIL, and soot steadily accumulates, eventually triggering DPF restriction codes and an engine derate if not repaired.

Common causes

  • Harness chafed against the exhaust or frame, shorting the control circuit to ground
  • Corroded, heat-damaged, or backed-out terminal at the doser injector connector
  • Failed injector solenoid coil with shorted windings
  • Open circuit in the injector supply or control wiring
  • Failed low-side driver in the ECM

Common misdiagnoses

  • DPF replaced or cleaned when the root cause is a doser circuit fault
  • Diesel oxidation catalyst condemned for failed regenerations
  • Exhaust gas temperature sensors replaced unnecessarily
  • Doser injector replaced when the wiring harness is actually at fault

Troubleshooting steps

  1. 1.Measure the injector coil resistance

    Disconnect the connector and compare coil resistance to spec (typically low single digits to about 15 ohms depending on OEM); near-zero indicates shorted windings.

  2. 2.Check the control circuit for a short to ground

    With both ends disconnected, an ohmmeter from the control wire to chassis ground should read open (OL).

  3. 3.Inspect the harness routing along the exhaust

    Look for melted insulation, chafe points at brackets, and heat damage near the DOC where these harnesses commonly fail.

  4. 4.Verify supply voltage and driver operation

    Confirm battery voltage on the feed circuit key-on, then command the injector with a bidirectional scan tool while watching for the driver to pull the circuit low.

  5. 5.Repair wiring or replace the injector and confirm

    Run a forced/stationary regeneration and verify commanded dosing produces the expected exhaust temperature rise without the code resetting.

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