Error Code P0491 is defined as Secondary Air Injection System Insufficient Flow Bank 1. This means there’s a problem on the side on the engine that holds the cylinder #1, usually caused by a bad air inlet, relay, fuse, pump or faulty wires.

This code is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II, or vehicles made since 1996 up to present. Specifications on the definition, troubleshooting and repairs of course, vary from one make and/or model to another.


The secondary air injection system injects ambient air into the exhaust to dilute the mixture and enable lower emission production. Basically, fresh air is pumped to the exhaust system during a cold start to feed both the oxygen and exhaust gases, and ensuring more complete burn and less harmful emissions from the vehicle.

Information about air pressure and air flow of this system is sent to the PCM (powertrain control module, also known as ECM or engine control module in other vehicles makes), which then converts the information to voltage signals. This is what allows the computer to determine whether there’s a problem or not. If the readings of the voltage don’t match the specifications from the manufacturer, the PCM will see it as an error/malfunction and will set the Error Code P0491.

Secondary air injection system is commonly found among Audi, BMW, Porsche and VW vehicles, and other vehicles. It features air pump, exhaust manifold, vacuum control switch, input check valve and an electrical input circuit for the vacuum control switch, along with tons of hoses.

Other related codes to secondary air injection system are P0410, P0411, P0412, P0413, P0414, P0415, P0416, P0417, P0418, P0419, P041F, P044F, and P0492.

Common Symptoms

Error Code P0491 doesn’t show a lot of symptoms, aside from the illumination of the Check Engine light and hissing sound from the air injection system, which is a sign of a vacuum leak. In some cases vehicles may show other symptoms, such as:

  • Stalling while idling or at start-up
  • Sluggish acceleration

Other symptoms related to other secondary air injection error code may also be present.

Possible Causes

There are multiple factors that may lead to this error code condition, such as:

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

How to Check

As with other trouble codes, mechanics start their diagnosis of P0491 by using an OBD-II scanner to verify the set code(s) and record the relevant data when the code was set. To verify that the code is not an intermittent condition, the codes as cleared and the vehicle will be taken to a test drive and scanned to see if the codes return.

Then, they will inspect the air injection pump’s wiring harness and look for signs of damage or shorts in the connections.

Next, they inspect the system hoses and fittings and look for signs of cracks, heat damage and other conditions that may cause the hoses to leak.

Then, they test system fuses.

After that, they will test that the one-way check valves to make sure that air only goes through one direction.

Next, they test the secondary air injection system pump.

How to Fix

Solutions for this error code are pretty straightforward, which include:

  • Replacement of damaged one-way check valve pump
  • Replacement of secondary air injection pump
  • Replacement of damaged connectors, wiring, and other electrical components
  • Replacement of damaged vacuum hoses or fittings that causes leaks
  • Replacement of pump fuses
  • Cleaning the cylinder head

Error Code P0491 is one of the less serious trouble codes out there. However, as with other trouble codes, it can pose potential danger to the vehicle as it can cause the vehicle to stall while driving.

Also, since secondary air injection system is directly related to emission production, malfunctions in this system can cause your vehicle to fail emission tests. Thus, it’s always wise to fix problems as soon as possible.