P0418 – Secondary Air Injection System Control “A” Circuit

P0418 is a diagnostic trouble code that points to a problem in your vehicle’s secondary air injection system, specifically the “A” relay control circuit. In plain terms, your engine computer is not happy with the electrical signal going to the air pump relay. While this code usually doesn’t leave you stranded, it can affect emissions, cold-start performance, and your chances of passing an inspection. Understanding what P0418 means, how it shows up, and how to test it will help you decide whether you can tackle it yourself or need a professional.

What Does P0418 Mean?

P0418 stands for “Secondary Air Injection System Relay ‘A’ Circuit Malfunction.” Your ECU commands the secondary air injection pump on through a relay when the engine is cold. The pump pushes fresh air into the exhaust to help burn off extra fuel and reduce emissions. When the ECU sees an incorrect voltage or no response from the relay “A” circuit, it sets P0418 and turns on the check engine light.

This code is focused on the electrical side of the secondary air system: the relay, wiring, fuse, and control signal. The pump itself may be fine or may also be damaged, but P0418 tells you to start with the relay circuit.

Quick Reference

• Code: P0418
• Definition: Secondary Air Injection System Relay “A” Circuit Malfunction
• System: Secondary Air Injection (SAI) / Emissions control
• Severity: Low to moderate (mainly emissions and drivability on cold start)
• Common Fixes: Replace relay, repair wiring, replace fuse, fix or replace air pump

Real-World Example / Field Notes

In the shop, I see P0418 most often on higher-mileage vehicles in rust-prone areas. A typical case: a sedan comes in with a check engine light and a failed emissions test, but the owner says it drives fine. Scan tool shows P0418 stored, sometimes along with another secondary air code. A quick check reveals a corroded relay socket and a blown fuse from a partially seized air pump. Replacing the relay and pump, cleaning the connector, and clearing the code usually restores normal operation and gets the car through inspection.

Symptoms of P0418

• Check engine light illuminated, sometimes the only obvious symptom.
• Failed emissions test due to stored code or incomplete readiness monitors.
• Rough cold start or slightly unstable idle right after startup.
• Increased cold-start emissions or fuel smell from the exhaust on first start.
• Secondary air pump noise change (no pump sound when cold, or loud/harsh noise).
• Possible reduced fuel economy in city driving, especially in cold weather.
• Other secondary air codes present, such as flow or pump performance faults.

Common Causes of P0418

Most Common Causes

• Failed secondary air injection relay “A” – internal contacts burned, stuck, or open.
• Blown secondary air pump fuse – often from a shorted or overworked pump.
• Corroded or loose relay socket – moisture and road salt corrode terminals.
• Damaged wiring to the relay – broken, rubbed-through, or shorted wires.
• Faulty secondary air pump – draws too much current, causing relay or fuse issues.

Less Common Causes

• ECU driver failure – the engine control unit cannot properly switch the relay on.
• Poor ground connection for the pump or relay, causing low voltage and erratic operation.
• Aftermarket modifications that remove or bypass the secondary air system.
• Water intrusion into the relay or fuse box from windshield or cowl leaks.
• Incorrect relay installed – wrong pin layout or rating after previous repairs.

Diagnosis: Step-by-Step Guide

To diagnose P0418 correctly, you should have a basic scan tool, a digital multimeter, a test light, and access to wiring diagrams for your specific vehicle. A power probe can help, but use it carefully. Safety glasses and a simple 12V jumper wire are also handy.

1. Confirm the code and check for companions. Connect a scan tool, read all stored and pending codes, and note if you have additional secondary air codes like P0410 or P0412. This helps narrow down whether it’s only the relay circuit or the whole system.
2. Check freeze frame data. Look at engine temperature, RPM, and voltage in the freeze frame. P0418 usually sets on a cold start. If the code sets under odd conditions, you may have an intermittent electrical issue.
3. Inspect the fuse and relay. Locate the secondary air pump fuse and relay “A” in the underhood fuse box. Visually inspect for corrosion, melted plastic, or signs of overheating. Test the fuse with a meter; don’t just look at it.
4. Swap or bench-test the relay. If there’s an identical relay in the box (for example, for the horn or A/C), swap them temporarily. If the problem moves, the relay is bad. You can also bench-test the relay by applying 12V to the coil pins and checking continuity across the switched pins.
5. Check for power and ground at the relay socket. With the key on, use a multimeter or test light to verify that the relay socket has battery power on the supply terminal and a solid ground where required. No power could indicate a wiring issue or upstream fuse problem.
6. Command the pump on with a scan tool. Many scan tools allow you to activate the secondary air pump through bi-directional controls. While commanding it on, check for voltage at the pump connector. If the relay clicks but no power reaches the pump, suspect wiring between relay and pump.
7. Test the pump current draw. If you can get the pump to run, use an ammeter to see how much current it pulls. A pump that draws excessive current can overheat the relay and blow fuses, leading to P0418. Compare to spec if available; if not, watch for obviously high draw or breaker tripping.
8. Inspect wiring harness and connectors. Follow the harness from the relay to the pump and grounds. Look for rubbed-through insulation, green corrosion, or broken wires, especially near the front of the vehicle where moisture and debris collect.
9. Check ECU control signal (advanced). If power, ground, and the relay all test good, use a meter or scope to verify that the ECU is sending a control signal to the relay coil when commanded. No signal may point to an ECU fault or a break in the control wire.

Pro tip: On many vehicles, Mode $06 data can show secondary air system performance test results. Checking these can help you see if the system is marginal or completely inoperative, even before a hard fault sets.

Possible Fixes & Repair Costs

Most P0418 repairs involve fixing the electrical path that controls the secondary air pump. Depending on what you find during diagnosis, the repair could be simple or more involved.

• Replace secondary air relay “A” – quick and common; parts usually $20–$60, labor minimal.
• Replace blown fuse and address cause – fuse is cheap, but you must fix the underlying issue (often a failing pump).
• Repair or replace damaged wiring/connector – can range from a simple terminal repair ($50–$150) to harness section replacement ($150–$400+).
• Replace secondary air injection pump – parts often $150–$450, labor $100–$300 depending on location.
• ECU repair or replacement (rare) – can run $500–$1,200+ with programming.

Typically, you’re looking at around $100–$350 for straightforward relay and wiring fixes, and $300–$800 if the pump also needs replacement. Costs vary with vehicle make, labor rates in your area, and whether you use OEM or aftermarket parts.

Can I Still Drive With P0418?

In most cases, you can still drive with P0418 without immediate risk of engine damage. The secondary air system mainly affects emissions during cold start, not basic engine operation. However, you may notice a rougher start and a persistent check engine light. The bigger concern is emissions testing: many states will fail your car if this code is present or if the secondary air monitor will not set to “ready.” So while it’s not an emergency breakdown issue, it’s not something you should ignore for long.

What Happens If You Ignore P0418?

Ignoring P0418 long-term can lead to increased emissions, repeated inspection failures, and potential damage to related components like the pump or relay from ongoing electrical stress. It also masks the check engine light, so you might miss new, more serious problems because the light is already on.

Last updated: March 1, 2026

Vehicles Commonly Affected by P0418

P0418 shows up across many brands, but it’s especially common on certain models that use electric secondary air pumps mounted low in the engine bay. You’ll frequently see this code on:

• GM vehicles – Chevrolet, GMC, Buick, and Pontiac trucks and sedans.
• Toyota and Lexus – especially V6 and V8 engines with complex SAI systems.
• Volkswagen and Audi – 1.8T, 2.0T, and some V6 engines with underhood pumps.
• BMW and Mini – older models with electric secondary air pumps near the front of the engine.
• Subaru – certain 2.5L engines with known secondary air system issues.

That said, any OBD-II equipped vehicle with a secondary air injection system can potentially set P0418 when the relay circuit acts up.