P0491 - Secondary Air Injection System Insufficient Flow Bank 1
The secondary air injection system pumps fresh air into the exhaust stream during cold starts to reduce emissions. Code P0491 indicates that the engine control module (ECM) has detected insufficient airflow from the secondary air injection system on bank 1. This is often caused by a failed air pump, a blocked or leaking hose, a faulty check valve, or a clogged combination valve. The system may also fail due to electrical issues like a blown fuse or relay.
01. Most Common Causes
- Failed secondary air pump
- Blocked or leaking air injection hoses
- Faulty check valve or combination valve
- Blown fuse or defective relay
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Failed emissions test
- Possible rough idle or hesitation during cold start
03. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm P0491 and check for related codes.
Step 2: Inspect the secondary air pump fuse and relay; replace if blown.
Step 3: Listen for pump operation at cold start; if silent, test pump power and ground.
Step 4: Visually inspect hoses for cracks, blockages, or disconnections.
Step 5: Test the check valve and combination valve for proper operation.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
Yes, but the vehicle may fail an emissions test and the check engine light will remain on. It is safe to drive short distances, but repair is recommended to avoid potential catalytic converter damage.
How much does it cost to fix?▼
The cost varies depending on the root cause. A simple fuse replacement costs around $10-$20, while a new air pump can range from $150-$400 including labor. Diagnostic fees are typically $50-$150.