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P0418Secondary Air Injection System Relay "A" Circuit Malfunction

There is an electrical fault in the control circuit for the primary secondary-air-injection pump relay (Relay A).

Definition

The PCM energizes the secondary air injection pump through a relay, and it monitors the Relay "A" control circuit for opens, shorts, and implausible feedback. P0418 sets when that control circuit does not behave as commanded — most commonly an open relay coil, a wiring fault, or a relay contact problem, and on many GM applications the root cause is a seized or water-damaged air pump overloading the circuit. The engine runs normally, but the MIL illuminates, the pump may never run (or run constantly, draining the battery), cold-start emissions rise, and the vehicle will fail an emissions inspection.

Common causes

  • Failed AIR pump relay (open coil or burned contacts)
  • Seized or water-damaged air pump overloading the relay and fuse
  • Open or shorted relay control wire between PCM and relay
  • Corroded relay socket or fuse box terminals
  • Blown fuse on the relay load side
  • Faulty PCM driver (rare)

Common misdiagnoses

  • Air pump replaced when only the relay or its socket was faulty
  • Relay replaced repeatedly while a seized pump kept overloading the circuit
  • PCM replaced for a corroded relay-socket terminal
  • Switching valves replaced for a relay-circuit code

Troubleshooting steps

  1. 1.Swap or bench-test the relay

    Test the relay coil (typically 50-120 ohms) and contacts, or substitute an identical known-good relay, and see whether the pump runs during a commanded AIR test.

  2. 2.Check fuses and measure pump current

    Verify the pump fuse is intact, then measure pump current draw when energized — a seized or water-filled pump draws well above spec and is the classic root cause.

  3. 3.Test the relay control circuit

    With the relay removed, confirm the PCM pulls the control terminal to ground (or supplies voltage, per the diagram) when the AIR test is commanded.

  4. 4.Inspect the relay socket and wiring

    Look for melted or corroded terminals in the underhood fuse box socket and chafed wiring between the relay, pump, and PCM.

  5. 5.Drain and inspect the pump

    Remove the pump inlet/outlet and check for water inside; a pump that has ingested condensation should be replaced along with any failed check valve that let moisture in.

  6. 6.Verify the system end-to-end

    Command the AIR system on, confirm the pump runs and airflow is diverted, then complete a cold-start monitor cycle.

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