P0491 Secondary Air Injection System Insufficient Flow (Bank 1)

P0491 indicates insufficient secondary air injection flow on Bank 1. The controller expected a Bank 1 oxygen sensor or pressure response when air was commanded into the exhaust.

Code details

Generic OBD-II · P0

System: Emissions
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace Bank 1 oxygen sensors for P0491 until secondary air flow to that bank is verified.

Symptoms of P0491

Check-engine light, often with no obvious change in how the car drives.
A failed emissions test or incomplete readiness monitors.
Sometimes a faint fuel-vapour smell (on EVAP codes).

Common causes of P0491

A worn catalytic converter or an oxygen/air-fuel sensor reading out of range.
EVAP leaks — a loose or failed gas cap, cracked hose, or purge/vent valve.
Exhaust leaks, fuel-trim problems, or a fault in a related monitor.

Useful checks before replacing parts

Review secondary air pump command, Bank 1 oxygen sensor response, pressure data, cold-start temperature, and freeze-frame monitor conditions.
Inspect the air pump, Bank 1 hoses, check valves, switching valves, blocked cylinder-head ports, vacuum control, and water intrusion.
Confirm actual air reaches Bank 1 and that the oxygen sensor indicates the expected lean shift during commanded air injection.

Related OBD-II codes

These nearby emissions entries can help compare wording and pin down the fault area.