Can I keep driving with P0418?

Yes. The secondary air system only runs for a short period at cold start and has no role once the engine is warm, so a relay fault has no effect on driveability or safety. The reasons to fix it are emissions compliance and clearing the check engine light. There is no mechanical urgency from a daily-driving standpoint.

Is P0418 just a bad relay?

Often, but not always. A failed relay is the single most common cause, and swapping it is the cheapest first test. But the same code can come from corroded sockets, water in the pump connector, broken wiring, a blown fuse, or an air pump that has seized or drawn enough current to burn the relay out. If a new relay fails again quickly, have the air pump's current draw tested before buying another relay.

Will P0418 cause an emissions test failure?

Yes. P0418 is an emissions-related code, and an active secondary air fault prevents the cold-start readiness monitor from completing. A vehicle with a stored or active P0418 will fail an OBD-II emissions inspection until the fault is repaired and the monitor runs to completion.

Why do secondary air pumps and relays fail so often?

The air pump usually mounts low in the engine bay where it is exposed to road splash and moisture, and the relay switches a high-current motor hundreds of times over the life of the car. Water intrusion corrodes the pump and connectors, and the heavy switching duty wears out relay contacts. On many vehicles these are known wear items rather than one-off failures.

OBD-II trouble code

P0418: Secondary Air Injection System Relay A Circuit Malfunction

The engine computer detected an electrical fault in the relay that powers the secondary air injection pump. The pump can't be energized to push cold-start air into the exhaust, so the check engine light is on — but the car still drives normally.

Quick facts

System: Powertrain
Category: Emissions / Secondary Air
Severity: Low severity
Drivable: Usually safe to drive short-term
Repair cost range: $50 – $800
DIY difficulty: Intermediate DIY

What does P0418 mean?

The secondary air injection (SAI) system pumps fresh air into the exhaust stream for the first 30 to 90 seconds after a cold start. That extra oxygen lets unburned fuel keep combusting in the exhaust, which heats the catalytic converter quickly and cuts cold-start emissions. The pump is a high-current electric motor, so the ECM doesn't power it directly — it energizes a relay, and the relay carries the heavy current to the pump.

P0418 means the ECM has detected a fault in the control circuit for that secondary air relay ('A' is the primary relay where more than one exists). The ECM watches the relay's control side electrically: when it commands the relay on, it expects to see the coil draw current and the switched side change state. When the feedback doesn't match the command, P0418 sets. The most common real-world causes are a failed relay, a corroded relay socket, or a break or short in the wiring between the ECM, the relay, and the pump.

Because the SAI system only runs briefly at cold start and does nothing once the engine warms up, P0418 has no effect on how the car drives. The pump simply won't run, the cold-start emissions monitor won't complete, and the check engine light stays on. The practical consequences are an emissions-test failure and the illuminated light, not a driveability or safety problem.

Common causes

Failed secondary air injection relay (burnt contacts or open coil)
Corroded or loose terminals at the relay socket
Open or shorted wiring between the ECM, relay, and air pump
Blown fuse on the secondary air pump circuit
Water intrusion into the relay or pump connector (common where the pump mounts low)
Failed air pump motor drawing excess current and damaging the relay
Poor ground on the relay or pump circuit
Failed ECM relay driver (rare)

Symptoms

Check engine light is on
Usually no driveability symptoms at all
Air pump does not run during the cold-start period
No audible pump whir on a cold start when the pump should be active
Failed emissions inspection because the secondary air monitor will not complete
Occasionally a blown fuse on the secondary air circuit

Diagnostic steps

1.Scan for companion codes. P0410, P0411, and the switching-valve codes (P0412-P0414) often appear with P0418 and help pinpoint whether the fault is on the relay/pump side or the valve side.
2.Locate the secondary air injection relay (often in the underhood fuse/relay box) and the air pump itself, then check the related fuse.
3.Inspect the relay socket and the pump connector for corrosion, melted plastic, and water intrusion — the pump frequently mounts low where it picks up moisture.
4.Swap the secondary air relay with an identical relay from a non-critical circuit and recheck, or bench-test the relay's coil and contacts.
5.With the relay commanded on by a scan tool, verify battery voltage reaches the pump motor and that the ground side is solid.
6.Measure the air pump motor's current draw against spec — a motor pulling too much current will burn out a replacement relay.
7.If the relay, wiring, fuse, and pump all check out, suspect the relay-control circuit back to the ECM before condemning the module.

Repair cost

$50 – $800

If the relay alone is at fault, the part is $20-$80 and the shop visit is typically $80-$150. Wiring or connector repair runs $100-$300 depending on access. If the air pump itself has failed and taken the relay with it, a pump replacement is the upper end at $300-$800 in parts and labor. Confirm whether the relay, wiring, or pump is the root cause before buying parts.

Estimate your repair

Run the numbers for your vehicle

Open the Repair Cost Estimator with secondary air injection pump replacement preselected. Adjust labor rate and vehicle category to fit your situation.

Estimate your repair cost

DIY vs shop

This is an intermediate DIY job. It usually involves diagnostic steps, specialty parts, and some careful work in tight spaces. If you have the tools and a service manual or trustworthy video for your specific vehicle, it is achievable in a weekend. Otherwise, a competent independent shop will be faster.