Secondary Air Injection System Insufficient Flow (Bank 1)
The P0491 code means the vehicle's onboard diagnostic system has detected: Secondary Air Injection System Insufficient Flow (Bank 1). This is a low severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- moderate
- Estimated cost
- $200 - $700 (parts and labor)
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Common Symptoms
- Check Engine Light on
- Failed emissions test
- Sulfur smell during cold start
- Slightly rough cold start idle
Probable Causes (Ranked by Likelihood)
- Clogged secondary air injection passages or check valve Very High
- Weak or failing secondary air pump High
- Leaking or stuck secondary air check valve Moderate
- Cracked or disconnected secondary air hose Moderate
Common Fixes by Vehicle
What techs usually find when diagnosing P0491 on specific vehicles — tap your vehicle for the fix and the exact part:
2005-2010 Honda Odyssey/Pilot/Ridgeline 3.5L moderate DIY
Honda P0491 is almost always the air injection solenoid or the air switching valve stuck. Clean or replace the combination valve on top of the exhaust manifold (P/N 36281-RYE-A01). Also inspect the silicone hose for cracks. Your dealer can confirm whether an open service bulletin or warranty extension applies to your VIN.
Labor: 2-3 hours2006-2014 Subaru Forester/Legacy/Outback 2.5L moderate DIY
Subaru Secondary Air Injection pump fails after water intrusion -- the pump lives in the fender well and rusts internally. Replace the pump (Subaru) and install the updated rubber duckbill check valve. Leaves & debris guard kit (Subaru) prevents repeat failure. Confirm the exact part for your year and engine with a retailer's fitment lookup or your VIN before ordering - these applications split by engine and build date.
Labor: 2 hours2003-2008 Honda Accord/Element 2.4L K24 easy DIY
On older Hondas with EGR-style air injection, carbon buildup plugs the air passages in the cylinder head. Solvent soak + carbon cleaner (Seafoam intake treatment) sometimes restores flow. If blocked solid, the head has to come off for decarbon -- a costly fix.
Labor: 30 min (chemical clean)Frequently Asked Questions
What is the secondary air injection system?
The secondary air injection system pumps fresh air into the exhaust manifold during cold starts. This extra oxygen helps the catalytic converter reach operating temperature faster, reducing cold-start emissions significantly.
Is P0491 a serious code?
P0491 is not typically a serious drivability concern. The secondary air system only operates during cold starts for a short period. However, it will cause a failed emissions test and should be repaired.
Why does my car smell like rotten eggs during cold start?
When the secondary air system is not providing sufficient airflow, the catalytic converter takes longer to heat up. During this extended cold-start period, hydrogen sulfide (rotten egg smell) may not be fully converted by the cold catalyst.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
-
Listen for the secondary air pump running during cold start; it should run for 30-90 seconds
-
Check airflow output from the secondary air pump; compare to specification
-
Inspect secondary air check valves for proper operation; they should allow airflow in one direction only
-
Check all secondary air hoses for cracks, disconnections, or blockage from carbon
Sources 1
This page is built from documented references. Verify against your own service info before repair work.
- SAE J2012Diagnostic Trouble Code Definitions (defines the generic OBD-II code ranges)
Commonly Affected Vehicles
- 2006-2016 BMW 3 Series
- 2008-2018 Toyota 4Runner
- 2007-2017 Lexus IS350
- 2010-2020 Subaru WRX
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