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.
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.
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.
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.
Here are the common triggers of the P0491 code:
The common symptoms associated with code P0491 include:
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.
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.
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.
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.
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.
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.
A faulty air pump or a leaking vacuum hose can trigger DTC P0491 and cause the air pump to make abnormal noises. These noises can get irritating fast, so find and replace the problematic parts as soon as possible. If you’re looking for quality replacement parts, check out CarParts.com.
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Don’t suffer through your vehicle’s noises for longer than you need to. Place your order for a new air pump or other replacement parts at CarParts.com today.
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