Code P0491 on the 2024 Subaru Outback indicates insufficient air flow from the secondary air injection system to Bank 1. This emissions system helps the catalytic converter warm up faster during cold starts by injecting fresh air into the exhaust manifold.
Secondary Air System Operation
During cold starts, the ECM activates the air pump to inject ambient air into the exhaust manifold. This additional oxygen supports burning of rich exhaust gases, generating heat that warms the catalytic converter more quickly. The system typically operates for 30-90 seconds after cold start.
P0491 Bank 1 Specificity
On the Outback's boxer engine, Bank 1 is typically the passenger side. P0491 indicates the air flow to this bank is insufficient, while Bank 2 may or may not have issues (check for P0492). The system monitors flow through oxygen sensor response when air is injected.
Common Causes
Air pump failure (motor not producing adequate flow) is common. Blocked or restricted air injection passages from carbon buildup prevent air from reaching the exhaust. A stuck air control valve won't direct air properly. Damaged check valve allows exhaust back-pressure to restrict flow.
Subaru-Specific Considerations
Subaru's boxer engine layout means air injection components serve different banks. Ensure diagnosis targets the correct bank. Cold climate operation is hard on air pumps - moisture can damage bearings and motors.