P0418: Secondary Air Injection System Relay A Circuit Malfunction

What does P0418 actually mean?

The computer commands a relay to supply power to the secondary air injection pump and monitors that relay circuit for the correct response. This code sets when the relay circuit does not behave as commanded, which can mean the relay itself, its wiring, or the fuse that feeds it.

The code sits in the secondary air injection area and is reported by the secondary air injection monitor. It names the circuit or the condition where the computer saw a reading outside its expected range. It does not name a failed part, and several genuinely different faults set this same code, which is the whole reason the causes below are ordered rather than listed.

What people usually notice

• Failed emissions test
• Check engine light appearing shortly after a cold start
• No noticeable drivability symptom

Why does P0418 set?

The causes below are ordered by how often they turn out to be the answer, cheapest and most common first. That order matters more than the list itself: the expensive component named in the code text is very rarely the first thing to replace, and working down from the top is what stops a secondary air injection fault becoming an expensive guess.

Failed air pump relay is the one to rule out before anything else. Relays switch significant current for the pump motor and are a common wear item.

The order worth checking on P0418

Start with freeze frame. Coolant temperature at the fault confirms whether the system was tested in its correct cold start window. That snapshot is taken at the instant the code set, and it is the single most useful piece of evidence you have, which is also why clearing the code before reading it is such a costly habit.

Diagnosing this one properly means commanding a component and watching whether it responds, which a read-only tool cannot do at any price. An actuator test that commands the relay directly confirms whether power reaches the pump as expected.

The order worth working in

• 1. Failed air pump relay: Relays switch significant current for the pump motor and are a common wear item.
• 2. Blown fuse feeding the relay or pump: Worth checking before assuming the relay itself has failed.
• 3. Corroded relay socket connector: Corrosion at the relay socket produces the same symptom as a failed relay.
• 4. Chafed wiring between the relay and the pump: Wiring routed near the exhaust or engine movement can wear through over time.
• 5. Failed pump drawing excessive current and damaging the relay: A failing pump motor can draw enough current to damage the relay that feeds it.

Is it safe to drive with P0418?

Safe to keep driving, since this system only matters briefly after a cold start and does not affect ongoing engine operation.

This is a severity 1 of 4 code. It will not usually strand you, but it will fail an emissions test while it is present, and the underlying fault rarely gets cheaper by being left alone.

What does it cost to ignore P0418?

This system exists mainly to pass emissions and to help the catalytic converter warm up faster, so ignoring it usually means a failed emissions test rather than engine damage.

There is also the diagnostic cost of waiting. Freeze frame data, pending code history and the readiness monitor state all describe the fault as it was when it set, and all of it is lost the moment somebody clears the code to make the lamp go out.