P0418 OBD-II Trouble Code: Secondary Air Injection System Relay A circuit Malfunction

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P0418 is a diagnostic trouble code (DTC) for "Secondary Air Injection System Relay A circuit Malfunction". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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P0418 code definition

A P0418 trouble code is an indicator that a problem has occurred in the secondary air injection system. Some codes related to P0718 include P0410, P0411, P0412, P0413, P0414, P0415, P0416, P0417, and P0419 trouble codes.

What the P0418 code means

A P0418 trouble code is stored on the PCM when voltage readings from the secondary air injection system register a difference of more than 10 percent over manufacturer's specifications. In addition to storing the code and illuminating the Check Engine light on initial detection, some vehicle models require multiple ignition cycles before an indicator lamp is lit.

What causes the P0418 code?

The causes of a P0418 trouble code often occur during cold weather and include water entering the secondary injection valve system via a faulty one-way check valve. Some other causes include wiring and connectors becoming burnt by high-temperature engine exhaust and an overloaded circuit, resulting in a blown fuse.

What are the symptoms of the P0418 code?

Symptoms of this code include engine hesitation when accelerating and stalling at startup and while idling. Another indication of a problem with the secondary air injection system is abnormal noise generating from the components.

How does a mechanic diagnose the P0418 code?

Diagnosing a P0418 trouble code requires the mechanic to use a digital volt/ohmmeter and an advanced scanner. In addition to these tools, the mechanic also needs to perform the following steps when diagnosing:

  • Inspect the one-way check valves to make sure they are still working properly. This requires the mechanic to remove the check valve and blow through it both ways. If the check valve is working, air should only pass through the valve one way.
  • Inspect any wiring, connectors, and components for damage.
  • Visually inspect all secondary air injection system fittings and hoses. The mechanic should look for any cracking, breaking, or other damage.
  • Check the fuses associated with the secondary air injection system. The mechanic can use the manufacturer's service manual to determine fuse locations.
  • If the fuses blow after replacement, diagnose the secondary air injector pump for failure.
  • Clear the code and test drive the vehicle to see if the code returns.

Common mistakes when diagnosing the P0418 code

Mechanics oftentimes fail to drain excess water from the pump housing when replacing a damaged secondary air injection pump. This, in turn, leads to the pump refreezing upon exposure to cold temperatures and the pump binding up and failing again. This problem is further increased if a mechanic fails to inspect and replace a faulty one-way check valve, which allows moisture to buildup and the pump to fail again upon water allowed into the system freezing.

How serious is the P0418 code?

If this problem is not fixed before cold weather hits, the water in the system can freeze and bind the pump, causing it to fail. Failure of the secondary air injection pump leads to higher emissions from the affected vehicle.

What repairs can fix the P0418 code?

To fix a P0418 trouble code, the mechanic should perform the following repairs if the secondary air injection system wiring, connectors, or components are found to be faulty:

  • Replace the one-way check valves if they do not work properly.
  • Replace any damaged or faulty wiring or connectors.
  • Replace any damaged or cracked fittings or hoses on the secondary air injection system.
  • Replace any blown fuses associated with the secondary air injection system, looking for reoccurrence of the fuses blowing upon testing.
  • Replace the secondary air injection pump if further diagnosis determines it to be at fault.

Need help with a P0418 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

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Related questions

Check engine light in code P0418

The air pump fuse is a 30 amp fuse that powers the pump. The "trigger" for the relay comes from the ignition switch, through a 10 amp fuse. Have you checked to see if the "trigger" wire to the relay...

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