P0491 – Secondary air injection (AIR) system, bank 1 malfunction

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By Reinier (Contact Me)
Last Updated 2017-01-10
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0491 Secondary air injection (AIR) system, bank 1 malfunction
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Wiring, AIR solenoid, hose connections, mechanical fault

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Table of Contents

  1. What Does Code P0491 Mean?
  2. What are the common causes of code P0491 ?
  3. What are the symptoms of code P0491 ?
  4. How do you troubleshoot code P0491 ?
  5. Codes Related to P0491
  6. Get Help with P0491

What Does Code P0491 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 4000 F (2040 C), but since exhaust gas alone (as the heating mechanism) takes too long to heat up the converter, additional oxygen (and sometimes, additional fuel, depending on the application) is introduced to speed up the heating process. Once the converter is hot enough to initiate the conversion process(s), the process becomes self-sustaining, and the Secondary Air Injection is shut off.

In terms of operation, the Secondary Air Injection system consists of an air pump, various hoses and pipes that carry the injected air to the exhaust system or engine, one-way check valves in the pressure lines to prevent exhaust gas and water from pushing back into the pump, wiring and connectors, and sometimes a relay, but more commonly a fuse to protect the control circuit, and a pressure sensor to monitor the overall operation/performance of the system.

When a cold engine is started, the air pump is activated to inject atmospheric air into the exhaust to assist in heating up the catalytic converter. The oxygen sensors monitors the shift toward a lean air/fuel ratio, but depending on the application and sometimes the ambient temperature, the PCM may enrich the air/fuel ratio to further assist in heating the catalytic converter to the point where it starts to function. On most applications, this is monitored by the #2-oxygen sensor, which has the main purpose of monitoring the operation of the catalytic converter. Thus, when the catalytic converter comes into operation, the #2 oxygen sensor alerts the PCM that then shuts down the air pump.

In a fully functioning system, the heating process is accomplished in less than two minutes, so if the PCM detects that the catalytic converter is not being heated up sufficiently to become self-sustaining in the period specified by the manufacturer due to a failure of, or malfunction in the Secondary Air Injection System, it will set code P0491, and illuminate a warning light.

The image below shows a much-simplified, basic schematic of a typical Secondary Air Injection system. Note however that this image does not represent any specific system design or layout, since the actual appearance, location, and operation of many Secondary Air Injection system components vary greatly between applications. Always refer to the manual for the application being worked to identify, and locate all relevant parts and components.


What are the common causes of code P0491 ?

Typical common causes of code P0491 could include the following-

  • Defective check valves that let water through to freeze in the air pump during cold spells. Note that improved check valve designs have largely eliminated this problem.
  • Defective air pump
  • Damaged, shorted, burnt, disconnected, or corroded wiring and connectors.
  • Blown fuse (Where fitted)
  • Defective air pump relay
  • Vacuum leaks in systems that use vacuum to control check valves
  • Leaks in pressure hoses
  • Blocked air passages that prevent injected air from reaching the catalytic converter

What are the symptoms of code P0491 ?

Apart from a stored trouble code and an illuminated warning light, the typical symptoms of P0491 could include the following, but note that this code very rarely causes driveability issues or other symptoms once the catalytic converter has reached operating temperature-

  • Mechanical noises caused by damage to the air pump
  • Some applications may exhibit varying degrees of hesitation upon acceleration during the warm-up cycle
  • Some applications may exhibit a rough idle, or stall unexpectedly during the warm-up cycle

How do you troubleshoot code P0491 ?

NOTE #1: Be aware that the simple check valves used in the pressure lines of older air injection systems have largely been replaced  by valves that are more complicated. On some applications, check valves are operated electrically with solenoids, while on others, engine vacuum acts on a diaphragm to open the valve. In both cases though, a failure of one, or both, check valves can set code P0491.

NOTE #2: Not all manufacturers use the same terminology to describe the various components of the Secondary Air Injection systems of their products. Therefore, to avoid confusion, misdiagnoses, and the unnecessary replacement of parts and components, always refer to the manual for the application being worked on for the exact terminology used by that manufacturer.

NOTE #3: Apart from a repair manual and a good quality digital multimeter, a hand-held vacuum pump fitted with a graduated gauge will be most helpful in diagnosing this code.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

Step 2

Make sure that the engine is cold, and refer to the manual to locate and identify all relevant components. Also, determine the location, routing, function, and colour coding of all associated wiring, hoses, and vacuum lines for future reference.

Step 3

If the scanner has control functions, use it to command the air pump on; most pumps sound like vacuum cleaners when they run, so if the pump does not start, check that the fuse (if fitted) is not blown, or that the relay that controls the pump is working.

Replace the fuse if required, but if the relay is suspected of being the problem, test it in accordance with the instructions provided in the manual.

Step 4

If replacing the fuse does not start the pump, or if the pump does not start even though the relay and its control circuit check out, disconnect the wiring from the air pump, and apply a direct current to the terminals in the connector on the pump. On most applications, this current is equal to battery voltage, but always refer to the manual to determine the correct voltage, as well as on the correct procedure to apply the current.

If the pump starts when a direct current is applied, prepare to test the pumps’ wiring for ground connectivity, continuity, and resistance. Compare all obtained readings with the values stated in the manual, and make repairs as required to ensure that all electrical values fall within the manufacturer’s specifications. Note that the pump motor forms part of the control circuit, and that it must therefore be tested as well. Check the resistance/continuity of the pump motor (refer to the manual to determine the correct procedure), and replace the pump if the values obtained do not conform to the specified values. Clear all codes after repairs are complete, and rescan the system to see if any codes return.

NOTE #1: Most applications require the completion of a drive cycle before emissions codes can be cleared. Consult the manual for the application on this point, and be sure to complete the drive cycle EXACTLY as described. Moreover, since the Secondary Air Injection monitor only runs when the engine is cold, allow the engine and exhaust system to cool down (preferably overnight), before continuing with the diagnostic procedure.

NOTE #2: Any grinding, thudding, knocking, or whining noises when the pump starts are indicative of mechanical problems inside the pump or its motor that means the pump must be replaced, since they are generally not repairable.

Step 5

If the pump starts (and it does not give off weird noises), but the code persists, disconnect the pressure lines from the pump before the check valve, and check that the pump actually produces an airflow. If it does not, check for obstructions and restrictions in the pumps’ feed line. Remove all obstructions, and/ or replace the air filter element as required. If the pump does produce a flow of air, inspect the hoses leading to the check valve for leaks, cracks, splits, or other damage that could cause a pressure loss. Replace the hose(s) as required or reconnect the hoses if they are not to be replaced.

Step 6

If the hoses leading to the check valve are serviceable, prepare to check the operation of the valve itself. If the scanner has control functions, use it to command the valve open but if it does not react to the control input, use the multimeter to check for current in the connector when the control input is repeated. If the obtained reading conforms to the manufacturer’s specification, the wiring is OK, but the valve itself is defective and must be replaced.

NOTE: This test obviously applies only if the check valve is electrically operated, but as with other types of valve, disconnect the hose leading out of the valve to check that air actually flows through it when it is open.

Step 7

If the check valve is operated by the engine vacuum, disconnect the valve from the engine vacuum, and attach the vacuum pump to the valve, but disconnect the hose leading out of the valve. Draw a vacuum to open the valve, and check that air actually flows through the valve. However, be sure to check that the vacuum holds for at least two minutes- if the vacuum decays (however slowly) and the test equipment is not defective in any way, the valve is defective, and it must be replaced.

If the vacuum holds steady and air from the pump flows through the valve, inspect all the vacuum lines leading up to the valve for signs of damage that could cause a loss of vacuum. Replace damaged vacuum lines as required.

NOTE: If all vacuum lines check out, prepare to test the solenoid that controls the vacuum for the check valve. Refer to the manual to locate this solenoid, and follow the directions provided in the manual on the correct procedure to test the solenoids’ operation. Note that testing procedures vary between manufacturers, so be sure to follow the directions exactly to obtain the most accurate results. Replace the solenoid if any obtained reading does not fall within the manufacturer’s specifications.

Step 8

If the air pump works and delivers a positive airflow, the check valve works as intended, and all wiring, vacuum lines and pressure hoses are serviceable, locate the point at which the pressure lines attach to the engine. On some applications, this could be on the exhaust manifold while on others, this point might be on the cylinder head.

Regardless of where the pressure line attaches, disconnect it, and activate the air pump and the check valve to allow air to flow through the system. Bear in mind though that due to the air pump’s design, the pressure will not be high enough to use a gauge to actually measure the pressure; in most cases, merely testing the air flow by placing a finger over the outlet is sufficient to confirm that a positive air flow is present.

Step 9

Since the function of the check valve is to prevent water and carbon from the exhaust being sucked, or pushed in the air injection system, it is possible that the pressure line might be partially blocked between the attachment point on the engine and the check valve.

Remove the pressure hose from the engine, and use compressed air or water to clear out any obstructions or restrictions that might be present. Once the pressure hose is clear, reconnect it, but not yet at the attachment point on the engine. Before you reconnect the pressure hose to the engine, the operation of the pressure sensor in the system must be checked.

This is a common failure on VW/Audi applications, but be aware that although testing procedures vary between applications, a simple test of this sensor involves measuring the changes in the signal voltage it produces as the pressure in the system changes. Refer to the manual to identify the signal wire, and connect the multimeter.

If the open end of the pressure hose is closed off with a finger while the pump is running, the sudden rise in pressure will cause a spike in the signal voltage- if this does not happen, check the resistance, continuity, reference voltage, and ground connectivity of all wiring associated with the pressure sensor. Make repairs as required, and repeat this step to ensure that all electrical values fall within specifications.

If the wiring checks out, test the sensor itself as per the instructions in the manual, and replace it if it does not conform to the manufacturer’s specifications.

Step 10

If there is indeed a flow of air at the attachment point, and the air-flow pressure sensor is fully serviceable, there is a strong likelihood that the passages through which the air must flow into the exhaust system are blocked. This is relatively common, and in some cases, it might be possible to open the passages in the manifold by poking a piece of stiff wire through the hole.

However, this is not a guaranteed remedy and in many cases, it might be necessary to remove the manifold to be able to clear out the air passages. Note that on some applications, removing an exhaust manifold can be a daunting task even for professional mechanics, so non-professional mechanics that are not comfortable with the idea of removing the manifold should rather refer the vehicle to a repair shop to have this procedure performed.

The issue of clogged air passages is even more serious if the blockage is in the cylinder head. Removing a cylinder head is a task that is best left to professionals, which is why non-professionals are strongly urged NOT to attempt a task of this magnitude unless they have the required knowledge, experience, and equipment- and especially if the affected engine is fitted with a timing chain and/or variable valve/cam timing.

  • P0492 – Relates to “Secondary Air Injection System Insufficient Flow Bank 2”

Note that while P0492 is directly related to P0491 – “Secondary Air Injection System Insufficient Flow Bank 1”, many other generic codes such as P0411, P0412, P0413, P0414, P0415, P0416, P0417, P0418, P0419, P041F, P044F, P0491, and P0492 could cause P0491/P0491, or contribute to setting them. Thus, if any of the other codes listed here is present along with P0491/P0492, refer to the manual for the application being worked on for detailed information on the implications of the codes listed here for that application.

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