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P0491 is a diagnostic trouble code (DTC) for "Secondary Air Injection System Insufficient Flow Bank 1". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $79.99. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.
This is the OBD-II code for Secondary Air Injection System Insufficient Flow Bank 1, indicating a problem on the side of the engine which contains cylinder #1.
The secondary air injection system utilizes ambient air, injecting it into the exhaust, thereby diluting the mixture and facilitating lower emissions production. Messages about the air pressure and flow through this system are sent to the PCM (powertrain control module), which converts these messages into voltage signals. This makes it easier for the computer to determine whether there is a problem – if the voltage reading doesn’t match the manufacturer’s specifications, the PCM can tell there’s a malfunction somewhere along the line and stores the P0491 code.
Secondary air injection system insufficient flow may be caused by:
There may only be an illuminated Check Engine Light, but this DTC is typically accompanied by other signs as well. These may include a hissing sound from the air injection system (a sign of a vacuum leak), sluggish acceleration, or stalling while idling or at start-up. Other DTCs related to the secondary air injection system may also be present.
Diagnostic procedures may include:
Using an OBD-II scanner to verify the set code and record other relevant data
Verifying that it isn’t an intermittent condition by clearing the codes, road testing the vehicle, and scanning the computer again to see if they return
Inspecting the air injection pump’s wiring harness and other connections for damage or shorts
Inspecting the system hoses and fittings for heat damage, cracks, and other conditions that may be causing a leak
Testing system fuses
Testing the one-way check valves to ensure that air is only able to go through in one direction
Testing the secondary air injection system pump
If the secondary air injection system pump is replaced without inspecting the check valves, repeated pump failure will likely result. A thorough diagnostic procedure should be carried out on all related components.
This DTC is less serious than some, but it can potentially pose a danger if the vehicle stalls while you’re driving. In addition, since the secondary air injection system is directly related to emissions production, any malfunction within this system could cause your vehicle to fail emissions testing. In fact, just the Check Engine light being illuminated will likely cause your vehicle to fail, so it’s best to have this problem addressed as soon as it’s convenient for you.
Solutions for P0491 may include:
Cleaning the cylinder head
Technicians note that this malfunction almost always occurs in times when the weather is cold. The reason for this is because when the one-way check valve goes bad, water – condensation, rather – gets into the pump and binds it up, creating insufficient air flow.
In engines with a V configuration, there are typically dual inlet ports and thus, dual check valves. Make sure to check both of these valves to ensure that air can only move through one way.
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