The secondary air pump draws a lot of current, so the engine computer switches it with a relay. A P0418 code means the computer found a problem in the circuit for secondary air injection relay A.
This guide explains how the relay circuit works, what usually fails and how to test it without replacing the pump.
P0418 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 4 | Auxiliary emission controls | The fault involves emission control systems such as EGR, secondary air, EVAP or the catalytic converter. |
| 18 | Specific fault | Secondary Air Injection System Relay A Circuit Malfunction |
What the P0418 Code Means
The SAE J2012 standard defines the P0418 code as Secondary Air Injection System Relay A Circuit Malfunction. The secondary air injection (AIR) system pumps fresh air into the exhaust for the first 30 seconds to 2 minutes after a cold start. The extra oxygen burns leftover fuel in the exhaust, which heats the catalytic converter quickly and cuts emissions while the engine is cold.
A typical system has an electric air pump, a relay that powers it, one or more switching or check valves that open the air path and stop exhaust from flowing back, and hoses or pipes to the exhaust ports. The powertrain control module (PCM) runs the pump and opens the valves, often using a vacuum solenoid or an electric valve.
The PCM checks the system by watching the upstream oxygen sensors. When the pump runs, the sensors should read lean because of the extra air. On some vehicles, a pressure sensor in the air path does the same job.
Relay A is usually the main air pump relay. The PCM switches the relay coil, and the relay contacts send battery power to the pump. This code points to the circuit the PCM controls or monitors.
Symptoms of a Bad Air Pump Relay A
Most symptoms appear right after a cold start. Common signs of a P0418 code include:
- Check engine light on
- No pump noise after a cold start
- Pump running all the time if the relay sticks
- Failed emissions test
- Dead battery if the pump runs with the key off
A relay that sticks closed can run the pump constantly, which can burn out the pump and drain the battery.
Most Common Causes of the P0418 Code
These are the usual causes:
- Failed relay. An open coil or stuck contacts.
- Corroded relay socket. Common in underhood fuse boxes.
- Damaged control wire. Between the PCM and relay.
- Blown fuse. For the relay coil or pump.
- Failing pump. Too much current can damage the relay.
- PCM driver fault. Rare.
Tools You Will Need
You can do most of this testing at home with basic tools:
- OBD-II scan tool that can command the air pump and valves
- Digital multimeter and test light
- Hand vacuum pump for vacuum-operated valves
- Wiring diagram for your vehicle
- Flashlight for checking hoses and valves
How to Track Down the P0418 Code
Many secondary air tests need a cold engine, since the system only runs for a short time after a cold start. Let the vehicle sit for several hours before testing.
Swap or Test the Relay
Swap the relay with an identical one, such as the horn relay. If the code clears or the pump starts working, replace the relay. You can also measure the relay coil, often about 50 to 120 ohms.
Check the Relay Socket
With the key on, check that one coil terminal in the socket has battery power. Command the pump on and check that the PCM grounds the other coil terminal. Check the contact terminals for power and corrosion.
Check the Pump Current
If the relay keeps failing, measure pump current with a clamp-on amp meter. A pump that draws much more than its rating can burn relay contacts.
Check that the pump is not seized by commanding it on and listening for it to run.
Check for a Stuck Relay
With the key off, listen for the pump. If it runs, pull the relay. If the pump stops, the relay contacts are stuck closed.
Repairs That Fix the P0418 Code
Match the repair to what your tests found. Then clear the code and drive under the same conditions that set it.
- Replace a failed relay.
- Clean or repair a corroded relay socket.
- Repair the control wire.
- Replace a blown fuse after finding the cause.
- Replace a seized or high-current pump that keeps burning relays.
Use the correct relay part number. Relays that look the same can have different pin layouts or current ratings, and the wrong one can damage the circuit.
Common Mistakes to Avoid
These mistakes waste money or leave the real fault in place:
- Replacing the pump before swapping the relay
- Using a relay with the wrong rating
- Ignoring a pump that runs with the key off
What Repairs Cost
The cost to fix a P0418 code depends on the cause. This table shows typical ranges for parts and labor.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Air pump relay or fuse | $5 to $40 | $50 to $150 |
| Switching or check valve | $40 to $250 | $150 to $500 |
| Air injection pump | $100 to $500 | $300 to $1,000 |
| Hose or vacuum line | $5 to $40 | $50 to $150 |
| Wiring or connector repair | $10 to $40 | $100 to $300 |
Relays are cheap and usually plug in, so this is often one of the least expensive repairs on the secondary air system.
Is It Safe to Keep Driving with a P0418 Code?
Yes, in most cases. The secondary air system only works for a short time after a cold start, so a fault does not usually affect how the engine runs once it is warm.
The vehicle will likely fail an emissions test, and some engines enter a reduced power mode if the pump or valves fail. If you hear a loud whine or see smoke from the pump, have it checked soon.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0410 | Secondary Air Injection System Malfunction |
| P0411 | Secondary Air Injection System Incorrect Flow Detected |
| P0412 | Secondary Air Injection System Switching Valve A Circuit Malfunction |
| P2440 | Secondary Air Injection System Switching Valve Stuck Open Bank 1 |
| P0420 | Catalyst System Efficiency Below Threshold (Bank 1) |
| P0415 | Secondary Air Injection Switching Valve B Circuit |
| P0416 | Secondary Air Injection Switching Valve B Circuit Open |
| P0417 | Secondary Air Injection Switching Valve B Circuit Shorted |
| P0419 | Secondary Air Injection Relay B Circuit |
Fixing Secondary Air Relay A
A P0418 code means the PCM found a fault in the secondary air relay A circuit. Failed relays, corroded sockets and wiring problems cause most cases.
- Swap the relay with a known good one.
- Check the relay socket for power and ground.
- Measure pump current.
- Listen for a pump that runs with the key off.
Frequently Asked Questions
Can a Bad Relay Cause the P0418 Code?
Yes, and it is the most common cause. Relays wear out, and a stuck or open relay sets this code. Swapping it with an identical relay is a quick, free test.
Where Is the Air Pump Relay?
It is usually in the underhood fuse and relay box. The lid or owner’s manual shows its position. Some vehicles mount it near the pump instead.
How Long Does the Air Pump Run?
On most vehicles, it runs for about 30 seconds to 2 minutes after a cold start, then shuts off. You may hear a whirring or humming sound from the engine bay during that time. It does not run on a warm engine.
Why Do Air Pumps Fail?
Moisture is the main enemy. Water from condensation or road spray can get into the pump through its inlet or a leaking check valve, which rusts the motor and bearings. A check valve that lets exhaust back into the pump can also melt it.
Which Vehicles Use Secondary Air Injection?
Many vehicles from the late 1990s through the 2010s use it, including models from Toyota, Lexus, Volkswagen, Audi, BMW, Mercedes-Benz, GM and Porsche. Newer engines often meet cold start emissions rules with other methods, such as retarded timing and faster-heating converters.
Can I Remove the Secondary Air System?
Not legally for road use in most places. It is part of the emissions system, and removing it will set codes and fail an inspection. Repair the faulty part instead.