P20B9 — Reductant Heater A Control Circuit/Open

P20B9: Reductant Heater A Control Circuit/Open — what it means, the affected system and what to check before replacing parts.

Meaning, symptoms, causes and checks

Reductant Heater A Control Circuit/Open. Basic catalogue definition; diagnostic details have not yet been reviewed.

Affected system: Powertrain

Symptoms

  • An engine or emissions-system warning may appear.
  • If reductant heating is unavailable, some vehicles may impose emissions-related operating restrictions.

Possible causes

  • The electrical path to reductant heater A may be open, including a broken conductor or damaged connector.
  • The heater itself or, less often, its control circuit may be faulty.

What to check

  • Confirm P20B9 and related SCR faults; inspect heater wiring and connectors.
  • Check heater supply, ground and resistance using vehicle service instructions.
  • Consider controller diagnosis only after the heater and wiring have been tested.

Also written as: P20B9-00, P20B900 (some scan tools add the fault-type byte, e.g. -00).