P0491: Secondary Air Injection System Insufficient Flow (Bank 1)

What does P0491 mean?

Too little secondary air is detected on bank 1 during its monitor. The supply pump may run, but a blocked port, leaking hose, or check/control valve can stop air reaching the intended exhaust passages.

Generic OBD-II definition: Secondary Air Injection System Insufficient Flow (Bank 1). Confirm applicability in your vehicle service information.

Common causes

• Carbon-blocked bank 1 air passages
• Leaking air or vacuum hoses
• Faulty pump, check valve, or control valve

Symptoms

• Check engine light
• Often no obvious driving symptom
• Cold-start emissions increase

Diagnostic focus

If P0492 is present too, check shared pump supply first. A bank-1-only fault with good shared airflow makes that bank's valve and passages more relevant.

Can I keep driving with P0491?

Severity: Low If the light is steady and the engine runs normally, schedule diagnosis. Stop for smoke, a burning electrical smell, or severe rough running; those are not normal secondary-air symptoms.

If the check engine light is flashing or the car runs badly, stop driving and have it checked.

Typical fixes

• Check cold-start pump operation
• Inspect bank 1 air hoses and valve operation
• Test airflow and clean passages only when blockage is verified

Sources & review

• OBD-Codes.com: P0491
• TroubleCodes.net: generic code definitions

Definitions and diagnostic background checked 2026-11-06. Generic guidance, not a substitute for the service procedure for your exact year, engine, and transmission.