P0491 OBD-II Trouble Code: Secondary Air Injection System Insufficient Flow Bank 1

P0491 code definition

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.

What the P0491 code means

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.

What causes the P0491 code?

Secondary air injection system insufficient flow may be caused by:

• A bad one-way check valve in the air inlet
• Damaged wiring or connectors, or loose sensor connections
• A faulty system relay
• A faulty air injection pump or fuse
• A bad secondary air pressure sensor
• A significant vacuum leak
• Clogged secondary air injection ports

What are the symptoms of the P0491 code?

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.

How does a mechanic diagnose the P0491 code?

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

Common mistakes when diagnosing the P0491 code

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.

How serious is the P0491 code?

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.

What repairs can fix the P0491 code?

Solutions for P0491 may include:

• Replacing any damaged wiring, connectors, or other electrical components

• Replacing the secondary air injection pump

• Replacing the pump’s one-way check valve

• Repairing or replacing any damaged vacuum hoses or fittings which could be causing a vacuum leak

• Replacing pump fuses

• Cleaning the cylinder head

Additional comments for consideration regarding the P0491 code

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.

Need help with a P0491 code?

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