P0418 Code: Secondary Air Injection Pump Relay "A" Circuit — Symptoms, Causes, and Fixes

Quick Answer

• What it means: The P0418 code indicates that the Powertrain Control Module (PCM) has detected a malfunction in the electrical circuit for the Secondary Air Injection (SAI) pump relay "A".
• Severity: Low. It is generally safe to keep driving, but your vehicle will not pass an emissions test.
• Most common cause: A faulty secondary air injection pump relay or a blown fuse (often caused by a failing air pump that is drawing too much amperage).

What the P0418 code means

The official OBD-II definition for P0418 is "Secondary Air Injection System Relay 'A' Circuit."

To understand this, you need to know what the Secondary Air Injection (SAI) system does. When you cold-start your engine, it runs "rich" (more fuel, less air), which creates high levels of unburned hydrocarbons. The SAI pump acts like a reverse vacuum cleaner, injecting fresh, oxygen-rich air directly into the exhaust manifold or catalytic converter. This extra oxygen helps burn off those excess hydrocarbons and heats up the catalytic converter faster, drastically reducing cold-start emissions.

Because the SAI pump draws a significant amount of electrical current, the PCM uses a heavy-duty relay (Relay "A") to turn it on and off. The PCM monitors the voltage on this relay circuit. If the PCM commands the relay to turn on, but it doesn't see the expected voltage drop on the control circuit, or if it sees voltage when the relay should be off, it triggers the P0418 code and illuminates your Check Engine Light.

Common symptoms

Because the SAI system primarily operates for only a minute or two during a cold start to reduce emissions, you will rarely notice any drivability issues. Symptoms include:

• Illuminated Check Engine Light: This is the most common and often the only symptom.
• Failed emissions test: Since the emissions system is compromised, the vehicle will automatically fail state smog inspections.
• Abnormal noises on cold start: If the relay is stuck closed, the pump may run continuously, emitting a loud whining or vacuum-like noise.
• Engine hesitation on cold start: In rare cases, a malfunctioning emissions system can cause a slight hesitation before the engine warms up.

Common causes

Ranked from most to least likely, the causes of a P0418 code are:

1. Defective SAI pump relay: The internal contacts of the relay can wear out, burn, or stick over time.
2. Blown fuse: The circuit is protected by a fuse. If the pump draws too much power, the fuse will blow, severing power to the relay.
3. Failing secondary air injection pump: Water condensation often builds up inside the pump housing, causing it to rust or freeze. When the pump binds up, it draws excess amperage, blowing the fuse or burning out the relay.
4. Wiring and connector issues: Corroded connectors, chafed wires, or loose grounds in the relay control circuit.
5. Faulty PCM: Very rare, but a damaged driver inside the engine computer can fail to ground the relay.

How a technician diagnoses it

An ASE-certified technician will follow a logical electrical diagnostic path to isolate the failure:

1. Scan tool testing: The technician will connect an advanced bidirectional scan tool and pull all DTCs and freeze-frame data. They will then use the scan tool to manually command the SAI pump relay ON and OFF to see if it clicks.
2. Visual inspection: They will inspect the relay box for signs of heat damage, corrosion, or water intrusion.
3. Check the fuses: The technician will test the main maxi-fuse for the SAI pump and the smaller control fuse for the relay. If a fuse is blown, it's a strong indicator that the pump itself is drawing too much current.
4. Relay circuit testing: If the fuse is good, the technician will remove the relay and use a digital multimeter (DMM). They will check pin 30 for constant battery voltage and pin 85 (or 86) for ignition voltage. They will then check if the PCM is providing the ground trigger on the control circuit when commanded.
5. Amperage draw test on the pump: If the relay is bypassed and the pump turns on, the technician will measure the pump's amperage draw. A healthy pump usually draws 10–15 amps. If it pulls 25+ amps, the pump bearings are failing (usually due to water intrusion), and replacing just the relay will result in a repeat failure.
6. Check valves inspection: They will inspect the one-way check valves on the exhaust manifold. If these valves fail, exhaust gases and condensation blow backward into the pump, destroying it.

Typical repair cost

The cost to resolve a P0418 code depends entirely on whether the failure is isolated to the electrical circuit or if the pump itself has destroyed the circuit. Note: These are estimates including parts and labor.

• Relay and/or Fuse Replacement: $50 – $100. (If the fix is this simple, you're lucky. However, the pump should always be tested for high amperage draw).
• Wiring Repair: $100 – $250. (Finding and repairing a broken wire or corroded connector).
• Secondary Air Injection Pump Replacement: $250 – $600+. (If the pump has failed and taken out the relay/fuse).
• Check Valve Replacement: $150 – $300. (Often recommended alongside pump replacement to prevent repeat failures).

Can I keep driving with this code?

Yes, it is perfectly safe to continue driving your vehicle with a P0418 code. The Secondary Air Injection system does not affect the engine's mechanical performance, fuel economy, or safety systems. However, your vehicle will be polluting more on cold starts, your Check Engine Light will remain on, and you will not be able to pass a state emissions or smog test until it is fixed.

Related codes

If you have a P0418, you may also see these related secondary air injection codes:

• P0410: Secondary Air Injection System Malfunction
• P0411: Secondary Air Injection System Incorrect Flow Detected
• P0412: Secondary Air Injection System Switching Valve "A" Circuit
• P0419: Secondary Air Injection System Relay "B" Circuit

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