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DTC code page

P0418: Secondary Air Injection System Relay A Circuit Malfunction

Quick answer: The ECU detected a fault in relay A control for the secondary air injection system.

Drivers also search this fault as secondary air relay A circuit malfunction, P0418 relay A fault, air injection relay A problem.

Severity: medium Family: powertrain Related paths: 8
Meaning

What P0418 usually means

P0418 shifts the diagnosis upstream to relay control. The air pump may be healthy and the valves may be capable of moving, but if relay A does not energize correctly, the system never gets the electrical support it needs during the cold-start window. Common causes include a failed relay, weak relay socket terminals, corroded fuse-box connections, bad command signal from the controller, or wiring that drops voltage only when the pump load is applied.

Fast triage

Start here before chasing parts

  • Scan first: save freeze-frame and pending codes before clearing anything.
  • Confirm the complaint: compare the stored code with current drivability symptoms.
  • Use context: trims, live data, and related codes usually narrow the fault faster than guesswork.
  • Work simplest to hardest: leaks, connectors, maintenance items, and known patterns before expensive components.
Initial checks

What to check first

  • Check whether the pump actually receives power during cold start rather than just listening for relay clicks.
  • Inspect the relay and socket for heat discoloration or loose terminal tension.
  • Measure voltage drop on the feed side while the pump is commanded on.
Driving risk

Can you keep driving?

P0418 usually allows continued driving, but it leaves the cold-start system dead and can worsen electrical heat damage if a bad relay or socket is ignored.

Moderate urgency: This code often allows short-term driving, but the right fix usually comes faster when you diagnose it early instead of waiting for more codes.
Likely causes

Common causes behind this code

  • Relay A failed internally or sticks intermittently
  • Corroded or overheated relay socket terminals
  • Fuse-box or power-feed issue causing voltage loss under load
  • Controller command problem to the relay coil side
  • Pump current draw abnormal enough to stress the relay circuit

Cause phrases often tied to this code: bad relay, relay socket corrosion, fuse box issue, command failure, voltage drop under load.

Diagnostic order

Suggested workflow

  1. Confirm relay A is the correct control path for the pump or stage named by the vehicle.
  2. Test relay command, output, and voltage drop under load.
  3. Inspect socket terminals, fuse links, and shared power distribution points for heat damage.
  4. Check pump current draw so you do not replace a relay that is being killed by an overloading motor.
  5. Retest cold-start pump operation and monitor response after repair.
Avoid guesswork

Common mistakes

  • Replacing the pump when the relay feed is the actual reason it never ran.
  • Bench-testing the relay unloaded and missing socket voltage-drop problems on the vehicle.
  • Ignoring a high-current pump that immediately damages the replacement relay too.
Repair path

Practical fix guidance

  • Restore stable relay control and loaded voltage delivery to the secondary-air system, then confirm the monitor passes.
  • If the relay or socket overheated, inspect the pump current draw before closing the case.
  • Protect fuse-box repairs from moisture and heat recurrence.
Vehicle context

Affected brands in this MVP

Brand hubs help broaden internal linking now and can evolve into make-specific diagnostic notes later.

Aliases and common searches

English phrases tied to P0418

Useful when the driver knows the wording but not the exact DTC yet.

  • secondary air relay A circuit malfunction
  • P0418 relay A fault
  • air injection relay A problem
Related search intent

Queries this page can answer naturally

  • P0418 code meaning
  • what does P0418 mean
  • secondary air relay A circuit malfunction
FAQ

Quick questions about P0418

Can a bad relay alone cause P0418?

Yes. A failing relay or overheated socket can stop the air pump from receiving power even when the rest of the system is fine.

Why test voltage drop for P0418?

Because relay circuits can look electrically present with no load but collapse once the pump draws real current.

Should I replace the pump and relay together?

Only if testing shows the pump is drawing abnormal current or has also failed.