P0491

AIR System Insufficient Flow Bank 1

Q: Is it safe to drive with P0491?

Yes. Secondary air injection only runs briefly at cold start to reduce emissions, so the car drives normally. Fix it to pass emissions and turn off the light.

Q: Can I just clear P0491?

It clears, but returns after the next cold-start self-test if the pump, valve, or passages are still restricting airflow.

Secondary air injection (emissions) Generic OBD-II High

What it means

P0491 means the secondary air injection system isn't pushing enough air into the bank 1 exhaust during cold start. That extra air helps the catalytic converter heat up and burn off emissions quickly, so the usual causes are a failed air pump, a clogged passage, or a stuck valve. The car drives normally, but the fault will keep the emissions monitor from passing.

High

Can I keep driving? Yes — the car drives normally; this is a cold-start emissions fault, but it should be fixed to pass emissions and clear the light.

Real-world scan data

Seen in

scans in the past year

Found in

<0.1%

of scans

Figures reflect only diagnostic scans recorded through our own apps and tools. Cars are typically scanned because a symptom or warning light prompted it, so these counts describe our scan records — not the incidence of faults in any model, and not the quality, reliability, or safety of any vehicle. No comparison between manufacturers or models is implied. All trademarks belong to their respective owners; no affiliation or endorsement is implied.

Common symptoms

What you'll notice

Check-engine light on
Little or no change in normal driving
Occasionally a rougher-than-usual cold idle right after start-up
May fail an emissions/readiness test

Common causes

Most likely first

Failed secondary air pump or air pump relay/fuseLikely
Carbon-clogged or blocked air injection passagesCommon
Stuck or leaking air switching/diverter valveCommon
Failed one-way check valve letting exhaust back into the plumbingLess often
Cracked or disconnected vacuum/air hoses on bank 1Less often

How to diagnose & fix

Work through it in order

Read freeze-frame data and confirm the fault sets during the cold-start air injection test.
Listen for the air pump running at cold start; if silent, check its relay, fuse, and power/ground.
Inspect the air switching/diverter valve and check valve for sticking or exhaust contamination.
Check the air hoses and injection passages for blockage or carbon buildup on bank 1.
Replace the failed pump, valve, or hose; clear the code and re-run a cold-start cycle.

P0491 by make

Based on commonly reported failure patterns across owner and repair communities, not on our scan data.

BMW

Commonly reported on BMWs with secondary air pumps, which run for a short time after a cold start and can fail or clog. A failing pump is often audible as a loud whine at start-up.

Chevrolet

On older Chevrolet trucks and cars with an air injection pump, the pump or its check valves can fail or clog with carbon. Most of the time it only affects emissions, not driveability.

Toyota

On some Toyotas and Lexus models with secondary air injection, valves and pipes can clog or stick over time. It mostly affects cold-start emissions rather than how the vehicle drives.

Volkswagen

Commonly reported on VW and Audi models with a secondary air pump, where the pump, its relay or the check valve fails or clogs. Cold-start whining or no sound at all are useful clues.

FAQ

Quick answers

Is it safe to drive with P0491?+

Yes. Secondary air injection only runs briefly at cold start to reduce emissions, so the car drives normally. Fix it to pass emissions and turn off the light.

Can I just clear P0491?+