P0491
AIR System Insufficient Flow Bank 1
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.
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.
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?+
It clears, but returns after the next cold-start self-test if the pump, valve, or passages are still restricting airflow.
Related codes
Often seen with P0491
Read & clear P0491 yourself
Scan your car, confirm the code, and clear it once it's fixed — with OBD DashX on your phone.
✓ Verified name — SAE J2012 / ISO 15031-6Guidance reviewed by OBD DashX