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P0418: Secondary Air Injection System Relay A Circuit
Severity: Low If the light is steady and the engine runs normally, schedule diagnosis. Stop for smoke, a burning electrical smell, or severe rough running; those are not normal secondary-air symptoms.
What does P0418 mean?
The secondary-air relay A circuit is malfunctioning. The pump may fail to turn on even if its mechanical parts are sound, because its supply relay or relay-control circuit is not working.
Generic OBD-II definition: Secondary Air Injection System Relay A Circuit. Confirm applicability in your vehicle service information.
Common causes
- Faulty air-pump relay
- Blown fuse or wiring short
- Failed pump overloading the supply
Symptoms
- Check engine light
- No secondary-air pump operation
- Usually normal warm driving
Diagnostic focus
A failed pump can overload its relay or fuse. Find the cause of an overload instead of only swapping relays or fitting a higher-rated fuse.
Can I keep driving with P0418?
Severity: Low If the light is steady and the engine runs normally, schedule diagnosis. Stop for smoke, a burning electrical smell, or severe rough running; those are not normal secondary-air symptoms.
If the check engine light is flashing or the car runs badly, stop driving and have it checked.
Typical fixes
- Inspect the pump relay and fuse
- Test relay power and the control command
- Check pump current draw if a fuse has failed
Related codes
Secondary Air Injection System MalfunctionP0412
Secondary Air Injection System Switching Valve A CircuitP0491
Secondary Air Injection System Insufficient Flow (Bank 1)P0492
Secondary Air Injection System Insufficient Flow (Bank 2)
Sources & review
Definitions and diagnostic background checked 2026-11-06. Generic guidance, not a substitute for the service procedure for your exact year, engine, and transmission.