P0418 on Subaru: How to Diagnose & Fix
Secondary Air Injection System Relay A Circuit
1 Subaru model · 1 documented issue · 1 critical
P0418 on Subaru by model: symptoms, diagnosis and fix
On turbocharged Forester XT models, the relay controlling the secondary air injection (SAI) pump can fail in the closed position, leaving the air pump running continuously. Sustained operation can overheat the pump, which may melt and give off odor or smoke and possibly catch fire; the check engine light comes on when this happens. Subaru recalled about 100,000 turbo vehicles (Forester XT, Impreza WRX/STI, Legacy/Outback turbo) for the defect.
Common Symptoms
- Burning / melting smell from engine bay
- Air pump audibly running after engine warmed up or off
- Check engine light with SAI codes
- Blown SAI-circuit fuse
How to Fix
Free recall repair under Subaru campaign WTM-73 (NHTSA 16V-738): the dealer replaces the secondary air injection pump relay. If the pump itself was already damaged by prolonged overheating it is replaced as well. Owners of turbo XT models should confirm the recall has been completed by VIN, as completion rates on older vehicles are often incomplete.
Sources for this issue (4)
- NHTSA Part 573 Safety Recall Report 16V-738 (Subaru WTM-73)recall
- Subaru Product Campaign Bulletin WTM-73 — Secondary Air Injection Pump Relay Replacementrecall
- Subaru Recall WTM-73: Power Relay May Fail Allowing Air Pump to Overheat (NHTSA 16V738000)recall
- Subaru WRX/STI, Forester XT, Outback Recall for Fire Risk Reaches US Modelsarticle
P0418 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0418 mean on a Subaru?
P0418 stands for "Secondary Air Injection System Relay A Circuit." This code means the Engine Control Module detected a fault in the control circuit for Secondary Air Injection (AIR) System Relay 'A'. The secondary air injection system pumps fresh air into the exhaust during cold starts to help the catalytic converter heat up faster and burn off pollutants, and Relay A switches power to that air pump. P0418 sets when the computer commands the relay but sees an abnormal voltage or no response on its control circuit. This is an emissions-related fault that may keep the air pump from running when needed. On Subaru it is documented across 1 model: Forester.
What are the most common causes of P0418?
The most common causes of P0418 are: Faulty secondary air injection relay (Relay A), Open, shorted, or corroded relay control wiring, Poor power or ground feed to the relay, Failed secondary air injection pump (drawing wrong current), Corroded relay socket or connector, Blown fuse in the air injection circuit, Faulty ECM relay driver. The specific cause varies by vehicle.
How much does it cost to fix P0418 on a Subaru?
Repair costs vary widely with the root cause and Subaru model; each issue on this page carries its own estimate where one is published.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.