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Summary
  • Diagnostic trouble code (DTC) P0491 stands for “Secondary Air Injection Insufficient Flow Bank 1.”
  • This code is logged once the PCM determines that the secondary air flow in bank 1 is too low. Bank 1 refers to the side of the engine where cylinder #1 is located.
  • An illuminated check engine light and unusual noises coming from the air pump are some of the symptoms associated with this trouble code.
  • A damaged air pump, a malfunctioning exhaust valve, and a clogged secondary injection port are some of the possible causes of a P0491 code.

Modern vehicles are equipped with several systems that work together to ensure emissions are kept to a minimum. For instance, the secondary air injection (SAI) system is a critical emissions system found on many vehicles.

That being said, it’s a must to ensure that the components in this system are in condition at all times. However, just like any other vehicle system, it isn’t uncommon for the SAI system to malfunction. P0491 is one of the several engine trouble codes related to a possible issue in the SAI system.

What Does the P0491 Code Mean?

Diagnostic trouble code (DTC) P0491 stands for “Secondary Air Injection Insufficient Flow Bank 1.” It is logged once the vehicle’s powertrain control module (PCM) perceives that there is a possible issue within the SAI system.

This code is logged once the PCM determines that the secondary air flow in bank 1 is too low. Bank 1 refers to the side of the engine where cylinder #1 is located.

See also  P0414 Code: Secondary Air Injection System Switching Valve “A” Circuit Shorted

To understand this code better, let’s discuss the main function of an SAI system. It provides the supply of air needed for the oxidizing process in the exhaust manifold or the catalytic converter. This system is also commonly called AIR, which means air-injection reaction.

mechanic running tests using a scan tool
P0491 code is logged once the vehicle’s powertrain control module (PCM) perceives that there is a possible issue within the SAI system.

A vehicle’s SAI system is equipped with an air pump. This pump may be driven by either the engine’s drive belt (older vehicles) or an electric motor (newer vehicles). Airflow from the pump is directed to either the catalytic converter or the exhaust manifold to aid in the oxidation of harmful exhaust gases. On most vehicles, the PCM manages a collection of solenoid-operated valves to direct airflow from the pump.

Once the PCM perceives that the secondary air flow in bank 1 is significantly low for a certain period of time, it will trigger the P0491 trouble code.

For more information on SAI that could help you fully understand P0491, read our technical discussion about the secondary air injection system.

Note: The definition of code P0491 may be different depending on the vehicle manufacturer. Consult the appropriate repair manual or repair database for the exact code definition.

What are the Possible Causes of the P0491 Code?

Here are the common triggers of the P0491 code:

  • Damaged air pump
  • Malfunctioning exhaust check valve
  • Circuit issues, such as damaged wires or poor connections
  • Leaking air hoses or vacuum hoses
  • Faulty vacuum control switch
  • Failed bypass valve or diverter valve
  • Clogged secondary air injection port

What are the Common Symptoms of the P0491 Code?

The common symptoms associated with code P0491 include:

How to Diagnose the P0491 Code

Diagnosing a P0491 code isn’t an easy task—especially for those who aren’t well-versed in auto repair. There’s a long list of issues that may trigger this code, so it may be difficult to identify which component is causing the problem.

See also  P2443 Code: Secondary Air Injection System Switching Valve Stuck Closed Bank 2

If you’re not an experienced DIYer, the best course of action is to take your vehicle to an auto repair shop. A mechanic would know how to diagnose the issue.

However, if you would really like to try and diagnose this code yourself, we recommend consulting vehicle-specific repair manuals or online repair databases. These resources usually contain information that can help you identify the right diagnostic procedures for your vehicle.

How to Fix the P0491 Code

Error code P0491 is a generic trouble code, which means it may be logged on various makes and models. However, that doesn’t mean that there is one generic solution that would work for all vehicles. For instance, repair procedures that worked for a P0491 on an Audi may not work for a P0491 on a BMW.

If you don’t have the right tools and know-how to fix this error code, take your vehicle to an auto repair shop. A mechanic would know how to resolve the problem.

However, if you’re a seasoned DIYer, and you prefer to fix this code yourself, we recommend consulting vehicle-specific repair manuals or subscribing to an online repair database.

A Closer Look at the Secondary Air (AIR) System

Keep in mind that not all engines use secondary air injection, so your vehicle may not even be equipped with it.

The secondary air injection system does nothing to help the engine run better, but helps tremendously with emissions, particularly in regard to adding oxygen where it’s needed to process carbon monoxide and hydrocarbons (CO and HC).

Early air injection systems appeared as a ‘70s era emissions component, complete with a belt-driven vane type air pump that delivers pressure to a special set of plastic switching valves through some rubber hoses.

See also  P2445 Code: Secondary Air Injection System Pump Stuck Off Bank 1
p0491 code secondary air injection system diagram
Secondary air injection system diagram | Image Source: Richard McCuistian

There are typically robustly capable one-way valves to prevent exhaust pulses from damaging the hoses and plastic valves. More recently, ECM/PCM controlled electric air pumps have replaced the belt-driven ones.

The plastic valves, either vacuum or electrically controlled, divert the air to the exhaust right as it leaves the engine during cold enrichment to neutralize the HC and CO produced while the engine is cold by adding extra O2 to bond with the hot, partially burned fuel (sort of like fanning dying embers in a fire). This heats up the exhaust at that point and helps bring the light-off catalyst online.

pcm controlled electric air pump diagram
More recently, ECM/PCM controlled electric air pumps have replaced the belt-driven ones. | Image Source: Richard McCuistian

As the engine warms, the air from a belt-driven pump is either exhausted through a muffler/filter to the atmosphere (on deceleration), or, when the engine is loaded (even idling), the air is directed downstream to assist the HC/CO cat process those exhaust gasses.

So, it helps the front (NOx) cat heat up more quickly and helps the rear cat do its work as well, depending on prevailing conditions.

A rationality check is typical where the ECM/PCM is checking the AIR system. It diverts air upstream while watching the O2 sensor(s). Air that is delivered upstream during deceleration can cause mild or severe backfiring in the exhaust.

Products Mentioned in this Guide

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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