Secondary Air Injection System Relay A Circuit Malfunction
The P0418 code means the vehicle's onboard diagnostic system has detected: Secondary Air Injection System Relay A Circuit Malfunction. This is a low severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- easy
- Estimated cost
- $30 - $200 (parts and labor)
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Common Symptoms
- Check Engine Light illuminated
- Air pump does not activate on cold start
- No drivability issues
- Failed emissions test
Probable Causes (Ranked by Likelihood)
- Failed secondary air pump relay Very High
- Open or shorted relay control circuit wiring High
- Corroded relay socket in fuse box Moderate
- ECM relay driver circuit fault Low
Step-by-Step Diagnostic Procedure
-
Locate the secondary air pump relay in the fuse box and swap it with a known-good relay of the same type
-
Check for power and ground at the relay socket with the key on
-
Verify the ECM is commanding the relay on during cold start using a scan tool
-
Inspect relay socket terminals for corrosion or heat damage
-
Check relay control circuit wiring from ECM to the relay socket
Common Fixes by Vehicle
What techs usually find when diagnosing P0418 on specific vehicles — tap your vehicle for the fix and the exact part:
2006-2014 Subaru Outback/Forester/Impreza 2.5L easy DIY
P0418 on Subaru is almost always a failed secondary air injection pump relay that sticks on or shorts. The relay is in the underhood fuse box labeled 'SAI PUMP'. Swap it with an identical relay from the box (like the horn relay) to confirm, then replace. If the relay is good, the pump itself may have failed from moisture ingestion -- check for rust at the air pump inlet hose.
Labor: 15 min2008-2013 BMW 328i/335i N52/N54/N55 moderate DIY
On BMW E90/F30 chassis, P0418 points to the secondary air pump relay located in the E-box (under the cowl, passenger side). Moisture corrodes the relay contacts. Replace the relay and check the air pump vacuum solenoid. BMW TSB SI B13 02 08 covers SAI diagnosis. The pump itself rarely fails -- it's almost always the relay or the rubber check valve hose.
Labor: 30 min2005-2011 Audi A4/A6 2.0T/3.2L easy DIY
P0418 on VAG 2.0T and 3.2L V6 indicates the SAI relay (#100 or #394) has failed. The relay lives in the E-box near the battery. Pull the relay and test continuity with 12V applied -- stuck-closed is the most common failure mode. Also scan for P0491/P0492 which confirm the diagnosis is airflow-related, not purely electrical.
Labor: 20 minFrequently Asked Questions
Can I drive with code P0418?
Yes. The relay controls the air pump which only runs during cold starts. Normal driving and engine performance are not affected.
How do I test the secondary air relay?
Swap it with another relay of the same type from the fuse box (such as the horn relay). If the air pump works with the swapped relay, the original relay has failed.
Is the relay an easy fix?
Yes. The relay is a plug-in component in the fuse box, typically costing $15-$40. It is one of the easiest repairs in the secondary air system.
Sources
This page is built from documented references. Verify against your own service info before repair work.
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