P0491 Insufficient flow of secondary air injection system, bank 1

P0491 – OBD-II Trouble Code Technical Description

Insufficient secondary air injection system flow (bank 1)

What does the fault code mean P0491?

The secondary air injection system is typically found on Audi, BMW, Porsche and VW vehicles and serves to inject fresh air into the exhaust system during a cold start. This allows for more complete combustion of harmful emissions. Code P0491 indicates a problem with this system, usually related to insufficient secondary air flow in bank #1, where bank #1 is the side of the engine with cylinder #1. The control system activates the air pump and controls the vacuum air injection mechanism. When it detects an inconsistency in the signal voltages, the PCM sets a P0491 code.

Possible reasons

Causes of the P0491 code may include:

1. Faulty check valve on the exhaust manifold.
2. The secondary air pump fuse or relay may be faulty.
3. Faulty air pump.
4. Suction hose leaking.
5. Bad vacuum control switch.
6. Closing the vacuum line.
7. Leakage in hoses/tubes between the secondary air injection pump and the secondary or combined air injection system.
8. The secondary air pressure sensor may be faulty.
9. The combination valve itself is faulty.
10. The secondary air injection hole in the cylinder head may be clogged with carbon deposits.
11. The secondary air holes in the cylinder head may become clogged.
12. Insufficient flow of the secondary air injection system can be caused by:
    • Bad one-way check valve on the air intake.
    • Damaged wiring or connectors, or loose sensor connections.
    • Faulty system relay.
    • Faulty injection pump or fuse.
    • Bad secondary air pressure sensor.
    • Significant vacuum leak.
    • Clogged secondary air injection holes.

What are the symptoms of a fault code? P0491?

Trouble code P0491 is usually accompanied by other symptoms, such as:

1. A hissing sound from the air injection system (a sign of a vacuum leak).
2. Slow acceleration.
3. Stopping the engine at idle or when starting.
4. Possible presence of other DTCs related to the secondary air injection system.
5. Malfunction indicator lamp (MIL) is on.

How to diagnose a fault code P0491?

Here are instructions for diagnosing error P0491:

1. Check the pump: Make sure the engine is completely cool. Remove the pressure hose from the pump or manifold check valve. Start the engine and check if the pump is pumping air out of the hose or outlet nipple. If air is pumping, go to step 4; otherwise, go to step 2.
2. Disconnect the electrical wiring connector from the pump: Apply 12 Volts to the pump using jumpers. If the pump works, go to step 3; otherwise, replace the pump.
3. Check the voltage supply to the pump: Make sure the engine is cool. Check the pump harness connector to ensure it has 12 volts by checking the voltage between the two pump harness plug terminals. If there is tension, repeat the first three steps to ensure the diagnosis is correct. If there is no voltage, check the fuses and relays.
4. Check the check valve: Make sure the engine is completely cool. Remove the pressure hose from the check valve. Check if air comes out of the hose when starting the engine. After the engine has been running for a minute, the valve should close. If it closes, then the check valve is working properly. If it doesn't close, go to step 5.
5. Check the vacuum switch: This will require a vacuum pump. Start the engine and hold the vacuum check valve nipple. If the valve is open, release the vacuum. If the valve closes, it is working properly. Otherwise, the problem may be with the vacuum switch.
6. Check vacuum pressure: Connect a vacuum to the control hose on the check valve. Start the engine. Make sure there is at least 10 to 15 inches of vacuum. Otherwise, additional diagnostics may require removal of some engine components.
7. Check vacuum lines and switch: Locate the vacuum switch on your vehicle. Check vacuum lines for damage, cracks or loose connections. If problems are found, replace the line.
8. Check manifold vacuum: Remove the inlet vacuum line from the control switch. Connect a vacuum gauge to the inlet hose to check the manifold vacuum while the engine is running.
9. Check the vacuum control switch: Apply vacuum to the vacuum control switch inlet nozzle. The valve must be closed and the vacuum must not be held. Apply 12 volts to the two terminals of the control switch using jumper wires. If the switch does not open and release vacuum, replace it.

This is a detailed instruction for diagnosing the P0491 error code.

Diagnostic errors

There are several mistakes a mechanic can make when diagnosing the P0491 trouble code. Here are some of them:

1. Incorrect diagnostic sequence: One of the most common mistakes is failure to follow the correct diagnostic sequence. For example, a mechanic might start by replacing components such as the secondary air injection pump without checking simpler, cheaper items such as vacuum hoses or sensors.
2. Failure to take into account environmental conditions: P0491 can be caused by various factors, such as cold temperatures. A mechanic may skip this aspect and try to diagnose the system under conditions that do not correspond to the problem.
3. Insufficient checking of vacuum components: Since vacuum is an important part of the secondary air injection system, the mechanic must take due care to inspect the vacuum hoses, valves, and vacuum sources. Missed vacuum leaks may be the cause of the P0491 code.
4. Not taking into account electrical problems: The P0491 code can also be caused by electrical problems such as broken wires, corroded connectors, or faulty relays. A mechanic should perform a thorough inspection of the electrical system before replacing components.
5. Lack of use of diagnostic equipment: Many modern cars are equipped with computers that can provide additional information about the problem. A mechanic who does not use diagnostic equipment may miss important data.
6. Insufficient communication with the owner: The mechanic may not ask enough questions of the vehicle owner that may help in identifying the circumstances that led to the P0491 code.
7. Replacement of components without confirmation of diagnosis: This is one of the most costly mistakes. A mechanic may replace components without being sure that they are causing the problem. This can lead to unnecessary costs and unrepaired malfunctions.
8. Insufficient documentation: Insufficient recording of diagnostic results and work performed may hinder future diagnosis and maintenance of the vehicle.

To successfully diagnose a P0491 code, a mechanic must follow a systematic and consistent approach, checking all possible causes and using diagnostic equipment to ensure the diagnosis is accurate and prevent unnecessary costs of replacing unnecessary components.

How serious is the fault code? P0491?

Trouble code P0491 is not usually a critical or emergency problem that will immediately result in vehicle breakdown or dangerous road conditions. It is linked to a secondary air injection system, which serves to reduce emissions and provide more efficient fuel combustion.

However, you should not ignore this code because it can lead to the following problems and consequences:

1. Increased emissions: Failure to comply with emission standards can impact the environment and may result in your vehicle not meeting emission standards in your region.
2. Decreased performance: If the secondary air injection system is not functioning properly, it may result in reduced engine performance and poor fuel efficiency.
3. Possible other problems: The P0491 code may be related to other problems or damage, such as vacuum leaks or electrical problems, which, if not corrected, can lead to more serious problems.
4. State Check Loss (MIL): When the P0491 code is activated, the Check Engine Light (MIL) will turn on on the instrument panel. If this code persists, the light will remain on continuously and you will not be able to notice other potential problems that may appear in the future.

Although P0491 is not considered an emergency fault, it is recommended that you have a mechanic diagnose and repair the problem. The problem may be relatively minor, but it is better to prevent it from getting worse and ensure reliable operation of the vehicle.

What repair will help eliminate the code? P0491?

Troubleshooting the P0491 trouble code may vary depending on the specific cause of this error. Here are some possible repair measures:

1. Replacing the air pump: If the air pump does not function properly, it needs to be replaced. This usually requires removing the old pump and installing a new one.
2. Replacing the check valve: If the check valve on the exhaust manifold is faulty, it should also be replaced.
3. Vacuum switch replacement: If the vacuum switch that controls the air system does not function properly, it should be replaced.
4. Checking and replacing vacuum hoses: Vacuum hoses may be leaking or damaged. They need to be checked and, if necessary, replaced.
5. Checking the secondary air pressure sensor: The secondary air pressure sensor may be faulty. It needs to be checked and replaced if necessary.
6. Checking wiring and electrical connections: Sometimes the problem may be related to the electrical connections or wiring. Check them for damage or corrosion and correct the problem if necessary.
7. Sediment cleaning: If the secondary air injection ports are clogged with carbon deposits, they can be cleaned to restore normal operation.

Repairs should be performed by a qualified mechanic as diagnosing and correcting problems with the secondary air injection system may require specialized equipment and knowledge. After the repair, you should also clear the P0491 error code and perform testing to ensure that the problem is successfully resolved.

What is P0491 Engine Code [Quick Guide]

P0491 – Brand-specific information

The P0491 code can occur on different makes of cars, and here is its definition for some of them:

1. Audi, Volkswagen (VW): Secondary air pump, bank 1 - low voltage.
2. BMW: Secondary air pump, bank 1 - low voltage.
3. Porsche: Secondary air pump, bank 1 - low voltage.
4. Chevrolet, GMC, Cadillac: Secondary air injection system, bank 1 - low voltage.
5. Ford: Secondary air injection (AIR) - low voltage.
6. Mercedes-Benz: Secondary air pump, bank 1 - low voltage.
7. Subaru: Secondary air injection (AIR) - low voltage.
8. Volvo: Secondary air injection (AIR) - low voltage.

Refer to your specific vehicle make and model for more detailed information about the problem and recommendations for troubleshooting P0491.