P0491
Secondary Air Injection System Insufficient Flow (Bank 1)
AIR system flow on bank 1 is below threshold during the cold-start self-test — typically a clogged check valve, leaking hose, or weak air pump.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Common causes
Ordered by how often each turns out to be the cause.
- Check valve clogged with carbonCommon
- Air pump weakPossible
Parts involved
- Secondary Air Injection Pump
- Secondary Air Check Valve
P0491 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.