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

What it means

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.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The secondary air injection system pumps fresh air into the exhaust during cold starts to help reduce emissions while the engine and catalyst warm up. The control module checks whether the expected airflow is present for Bank 1 during its cold-start test. P0491 sets when flow on Bank 1 falls below that threshold.

Common causes

Most likely first.

1. Check valve clogged with carbon
2. Air pump weak

Symptoms

• Check engine light on
• Rough running or misfire shortly after a cold start
• Increased exhaust emissions
• Failed emissions inspection

How to diagnose it

1. Read freeze-frame data and confirm the fault occurred during a cold-start test; check for related secondary-air codes that could point to a shared pump or supply problem.
2. Inspect the air pump, hoses, and Bank 1 air passages and check valve for loose connections, splits, blockage, or signs of exhaust soot.
3. Check whether the air pump operates during the cold-start test and inspect its electrical supply and relay circuit if it does not.
4. Test the Bank 1 check valve and air passage for restriction or leakage, and verify airflow using the manufacturer’s procedure.
5. Repair the confirmed restriction, leak, pump, or control fault, then verify airflow during a cold start.

Common fixes

• Reconnect or replace a split or disconnected secondary-air hose feeding Bank 1.
• Clear carbon from a restricted Bank 1 check valve or air passage if the component is serviceable.
• Replace a failed or weak secondary-air pump after confirming its supply and control are sound.
• Replace a faulty air control valve or relay if testing identifies it as the cause of low Bank 1 flow.

Can I keep driving?

The car is generally drivable if it runs normally, but the fault can increase cold-start emissions and may cause roughness or misfire just after starting. Avoid repeated short trips if possible and repair it before an emissions inspection.