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P20B9 Reductant Heater "A" Control Circuit/Open

The OBD2 code P20B9 indicates a malfunction in the control circuit for the reductant heater "A," which is responsible for heating the diesel exhaust fluid (DEF) to ensure proper operation.

Definition

The OBD2 code P20B9 indicates a malfunction in the control circuit for the reductant heater "A," which is responsible for heating the diesel exhaust fluid (DEF) to ensure proper operation. The "Open" part of the code suggests a break or fault in the electrical circuit, preventing the heater from functioning correctly

Common causes

  • Faulty reductant heater
  • Open circuit in the wiring
  • Short circuit in the wiring
  • Poor electrical connections
  • Failed power relay
  • Faulty engine control module (ECM)
  • Blown fuse related to the reductant heater circuit

Common misdiagnoses

  • Faulty reductant heater
  • Open or shorted wiring in the circuit
  • Defective engine control module (ECM)
  • Faulty temperature sensor
  • Issues with the diesel exhaust fluid (DEF) system
  • Problems with the reductant pump
  • Incorrectly installed or damaged connectors
  • Software issues in the ECM

Troubleshooting steps

  1. 1. Check for Additional Codes

    Use an OBD-II scanner to check for any other related trouble codes. This can provide more context and help identify if there are other issues present

  2. 2. Inspect the Wiring and Connectors

    Visually inspect the wiring and connectors associated with the reductant heater. Look for signs of damage, corrosion, or loose connections. Pay special attention to the harness that connects to the reductant heater

  3. 3. Test the Reductant Heater

    Use a multimeter to test the resistance of the reductant heater. Compare the readings to the specifications provided in the vehicle's service manual. If the resistance is out of range, the heater may be faulty and require replacement

  4. 4. Check the Control Circuit

    Inspect the control circuit for continuity. Use a multimeter to check for open circuits or shorts in the wiring. Ensure that the circuit is receiving the proper voltage from the engine control module (ECM)

  5. 5. Inspect the ECM

    If the wiring and heater are in good condition, the issue may lie with the engine control module. Check for any software updates or recalls related to the ECM. In some cases, reprogramming or replacing the ECM may be necessary

  6. 6. Check for Ground Issues

    Ensure that the ground connections for the reductant heater and control circuit are secure and free of corrosion. A poor ground can lead to circuit issues

  7. 7. Clear the Codes and Test Drive

    After performing the necessary repairs, clear the trouble codes using the OBD-II scanner and take the vehicle for a test drive to see if the code returns

  8. 8. Consult Technical Service Bulletins (TSBs)

    Check for any TSBs related to the P20B9 code for your specific vehicle make and model. Manufacturers may have issued guidance on common issues and fixes

  9. 9. Seek Professional Help

    If the issue persists after performing these steps, it may be necessary to consult a professional mechanic or technician who has experience with diesel exhaust systems and OBD-II diagnostics. By following these troubleshooting steps, you can systematically identify and resolve the issue associated with the P20B9 code