P0418 — AIR System Control A Circuit

P0418: AIR System Control A Circuit — what it means, the affected system and what to check before replacing parts.

Meaning, symptoms, causes and checks

AIR System Control A Circuit. Basic catalogue definition; diagnostic details have not yet been reviewed.

Affected system: Powertrain

Symptoms

  • Check-engine light; secondary air injection may fail its cold-start self-test.
  • The code identifies the relay control circuit, not proof that the air pump itself has failed.

Possible causes

  • Open fuse, defective air-pump relay or damaged relay coil/control wiring.
  • Corrosion, loose connector or an underlying electrical overload in the secondary-air circuit.

What to check

  • Confirm P0418 and record related secondary-air DTCs and freeze-frame.
  • Inspect relay and pump fuses without replacing a blown fuse repeatedly before finding the cause.
  • Check relay control and supply circuits using the specific wiring diagram.
  • Use an approved output test to establish whether the relay and pump respond correctly.
  • Repair the proven circuit or component fault, then repeat the relevant monitor.

Also written as: P0418-00, P041800, PO418 (some scan tools add the fault-type byte, e.g. -00).