Code Definition: P0491 - Secondary Air Injection System Insufficient Flow (Bank 1)
The P0491 code is a specific trouble code that indicates the secondary air injection system is not functioning properly, resulting in insufficient flow to the engine. The secondary air injection system is designed to inject fresh air into the exhaust stream to help reduce emissions during cold start conditions. When the system is not working correctly, the engine may have difficulty starting or running rough.
Possible Causes:
- Clogged or dirty secondary air injection pump
- Faulty or stuck secondary air injection valve
- Blocked or kinked secondary air injection hose
- Low battery voltage or poor electrical connection to the secondary air injection system
- Malfunctioning engine control module (ECM) or powertrain control module (PCM)
Symptoms:
- Difficulty starting the engine, especially when cold
- Rough engine idle
- Increased emissions
- Decreased fuel efficiency
- Illuminated check engine light
Diagnostic Procedure:
- Scan the vehicle for any other codes that may be present. If there are no other codes, proceed to step 2.
- Inspect the secondary air injection system for any signs of damage or blockages. Check the hoses, connections, and valves for any signs of wear or damage.
- Test the secondary air injection pump for proper operation. Use a scan tool to command the pump on and listen for the sound of the pump operating. If the pump is not working, check the fuse and relay to ensure they are properly connected and not blown.
- Test the secondary air injection valve for proper operation. Use a scan tool to command the valve open and closed. If the valve is not opening or closing properly, it may be faulty and need to be replaced.
- Check the battery voltage to ensure it is within the proper range. A low battery voltage can affect the operation of the secondary air injection system.
- If none of the above steps resolve the issue, suspect a malfunctioning ECM or PCM and consider reprogramming or replacing the module.
Repair Information:
- Replace the clogged or dirty secondary air injection pump with a new one. Be sure to use the correct replacement part number for your specific vehicle make and model.
- Replace the faulty or stuck secondary air injection valve with a new one.
- Replace the blocked or kinked secondary air injection hose with a new one.
- Check and clean the electrical connectors to the secondary air injection system to ensure good contact.
- Reprogram or replace the ECM or PCM if necessary.
- Clear the code and test drive the vehicle to ensure the problem has been resolved.
