What does it mean?
SPECIAL NOTES: While code P0491 indicates a relatively common fault on Secondary Air Injection systems, non-professional mechanics are nevertheless advised to read the section in the manual for the application being worked before attempting a diagnosis and/or repair of any Secondary Air Injection-related trouble code. There are two main reasons for this- the first being that the secondary air is not injected into the engine through the exhaust manifolds on all applications.
On some applications, the injected air is passed through passages in the cylinder head(s) that can clog up for various reasons. When this happens, the only way to resolve some Secondary Air Injection codes is to remove the cylinder head(s) from the engine to clear out the clogged passages.
The second reason has to do with the fact that in some cases, some applications may fail emissions tests repeatedly; even though there may be no Secondary Air Injection-related fault codes (or any other codes) present. In these cases, the PCM may indicate that the Secondary Air Injection system monitor is “NOT READY”, which means that one or more of the necessary required pre-conditions that is/are required for this monitor to run has not been met, or cannot be met.
Nine times out of every ten though, this monitor can be reset by completing a very specific drive cycle that is designed to allow the EVAP (Evaporative Emissions System) monitor, Oxygen Sensor monitor, and EGR (Exhaust Gas Recirculation) monitor to run and complete first, before the Secondary Air Injection monitor can run and complete.
The specifics of the required drive cycle will be provided in the manual, but be sure to complete ALL the steps in the specified order, and for the specified length of time to ensure that the monitor resets, or completes successfully. Thus, having at least a working knowledge of this system will enable non-professional mechanics to avoid many of the pitfalls and mistakes that come with diagnosing Secondary Air Injection system related codes.
It will also save hours of diagnostic time, and prevent the unnecessary replacement of parts and components. Note that due to the many significant design differences of Secondary Air Injection systems in use today, this guide cannot provide detailed diagnostic and repair information that will be valid for all applications under all conditions.
However, the generic steps outlined in the Troubleshooting section of this guide should enable most non-professional mechanics to diagnose and repair code P0491 successfully on most applications. END OF SPECIAL NOTES. OBD II fault code P0491 is a generic code that is defined by some manufacturers as “Secondary Air injection (AIR) system, Bank 1 Malfunction”, but sometimes also as “Secondary Air Injection System Insufficient Flow Bank 1”, by others.
Note though that both definitions have the same basic meaning, which is that the Secondary Air Injection system is either not injecting sufficient air into the exhaust system, or that it is not injecting any secondary air at all. “Bank 1” refers to the bank of cylinders that includes cylinder #1 on engines with two cylinder heads.
Note that while some applications will set code P0491 and illuminate a warning light on the first failure of the Secondary Air Injection System, others may require up to three successive failures before the code will be set, and a warning light illuminated.
The purpose of Secondary Air Injection systems during the 1970’s and 1980’s was to inject atmospheric air into the exhaust system to allow the oxygen in the air to aid in oxidizing, or burning off most of the unburned hydrocarbon molecules in exhaust gas. This process occurred mostly in the exhaust manifold but since the advent of catalytic converters, the oxygen in the injected air is used to speed up the heating of the converter greatly, which reduces exhaust emissions.
For a catalytic converter to start working, it must be at a temperature of at least 400 0 F (204 0 C), but s