P0418
UniversalSecondary Air Injection System Relay A circuit Malfunction
The secondary air injection system sends fresh air into the exhaust to help reduce emissions, often during engine warm-up. The control module monitors the electrical circuit for Relay A and has detected a fault in that circuit. This does not prove the relay itself is faulty; the issue may affect system operation or emissions and may turn on the check-engine light.
⚠️ Before You Begin (Safety First)
This fault is not usually an immediate driving hazard by itself. If the vehicle runs poorly, stalls, or shows other serious warnings, stop driving when safe and seek help. Avoid touching the exhaust or secondary-air components while hot.
Common Symptoms
- The check-engine light may be on because the control module detected a relay-circuit fault.
- The secondary air system may not operate as intended, potentially increasing emissions during the conditions when the system is used.
- An emissions inspection may be affected if the fault causes a test failure or prevents the relevant system monitor from completing.
- There may be no noticeable change in how the vehicle drives.
Common Causes
- Relay A may have an internal electrical fault.
- A loose, damaged, corroded, or disconnected relay or wiring connector may interrupt the circuit.
- A fuse, power supply, or ground associated with the circuit may be faulty.
- Wiring may be open, shorted, or damaged.
- The control circuit or its control module may have a fault.
- A fault in a connected secondary-air component could place an abnormal electrical load on the relay circuit.
Diagnostic Steps
- Check whether the vehicle has other warning lights or drivability problems; seek professional help promptly if it runs poorly or stalls.
- With the engine off and components cool, look for visibly loose, damaged, or corroded connectors or wiring around the secondary air system and its relay area.
- Check the owner’s manual for the correct fuse information and inspect only accessible fuses; replace a blown fuse only with the specified type and rating, and do not repeatedly replace one that blows again.
- Record any other diagnostic codes before clearing codes or disconnecting the battery, since they may help identify a shared electrical or system fault.
- If the fault remains, have the relay circuit and secondary-air operation tested using service information for the exact vehicle.
When to Call a Professional
Use vehicle-specific service information to identify Relay A and its circuit. A technician can check stored codes and scan data, command the relay where supported, and test the relay, wiring, connectors, power, ground, and control circuit. Confirm the secondary air system operates as intended before replacing parts.
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Commonly Asked Questions
Does P0418 mean the relay must be replaced?
No. It identifies a fault in the Relay A circuit, which may involve the relay, its power or ground, wiring, connectors, control circuit, or a connected component.
Can I keep driving with P0418?
If the vehicle drives normally and has no other serious warnings, this fault is not usually an immediate driving hazard. Arrange diagnosis because emissions performance may be affected. Stop when safe if the vehicle runs poorly or stalls.
What does the secondary air injection system do?
It supplies fresh air to the exhaust to help reduce emissions under conditions specified by the vehicle’s control system.
Can a visual inspection diagnose this code?
A visual check may reveal a loose connector or damaged wiring, but electrical testing may be needed to distinguish a relay fault from a circuit, power, ground, control, or connected-component problem.
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