P0491 – Secondary Air Injection System Insufficient Flow

What is the P0491 Code?

The OBD-II fault code P0491 means that the Secondary Air Injection System is not providing enough airflow into the exhaust system during cold engine start up on bank 1 (the side of the engine that contains cylinder #1).

Urgency and Severity

When you encounter code P0491, you can typically continue driving the vehicle cautiously. The car may exhibit symptoms like:

• Check engine light is on
• Engine may run rough during cold start
• Increased emissions

While this code is less severe than some, it could potentially cause the vehicle to stall while driving which poses a safety risk. It will also likely cause the vehicle to fail emissions testing. Therefore, it's best to diagnose and repair the issue as soon as convenient.

Code Applicability

P0491 is a generic powertrain code that applies to many makes and models of vehicles equipped with a secondary air injection system, particularly those with a V engine configuration. The specific components and repair procedures may vary between manufacturers.

System and Parts Involved

The secondary air injection system pumps fresh air into the exhaust stream to help reduce emissions during cold starts. Key components include:

• Secondary air injection pump
• Vacuum switching valves
• Check valves
• Vacuum hoses and lines
• Electrical wiring and relays

The system works by using the pump to inject air into the exhaust manifolds where it mixes with rich exhaust gases, allowing further combustion to reduce emissions. Sensors monitor the airflow and send readings to the powertrain control module (PCM).

Common Causes

Some of the most common causes for the P0491 code include:

• Faulty or clogged secondary air injection pump
• Leaking or damaged vacuum hoses
• Stuck open or damaged check valves
• Wiring issues like open circuits or shorts
• Faulty electrical connections
• Damaged or corroded wiring harness
• Failed PCM (rare)

Diagnosis Steps

To properly diagnose the root cause of the P0491 code, follow these steps:

1. Use an OBD-II scanner to verify the code and check for other related codes.
2. Visually inspect the wiring, hoses, and components of the secondary air injection system. Look for obvious damage, leaks, or poor connections.
3. Check the vacuum hoses for splits, kinks or leaks. Use a vacuum gauge to test for proper vacuum.
4. Test the electrical wiring and connectors for continuity and shorts using a multimeter.
5. Test the operation of the secondary air injection pump. Listen for unusual noises or lack of airflow.
6. Inspect the check valves to ensure they are allowing air to flow in only one direction.
7. If no other issues are found, the PCM may be faulty, but this is rare.

Repair Solutions

Repairs for the P0491 code may involve:

• Replacing the secondary air injection pump ($250-600 USD / €225-550 EUR)
• Replacing damaged vacuum hoses ($20-50 USD / €18-45 EUR)
• Replacing failed check valves ($40-130 USD / €35-120 EUR)
• Repairing wiring issues ($50-200 USD / €45-180 EUR)
• Cleaning electrical connections
• Replacing the PCM ($500-1400 USD / €450-1300 EUR)

Labor costs will vary but are typically in the range of $80-140 USD (€70-130 EUR) per hour with diagnosis and repair taking 2-4 hours on average. Total repair costs are usually $400-1200 USD (€350-1100 EUR).

Prevention

To help prevent the P0491 code from occurring:

• Regularly inspect vacuum hoses for wear and replace as needed
• Ensure electrical connections are clean and secure
• Address any other related engine issues promptly
• Follow recommended maintenance schedules for air and fuel filters

Associated Codes

Other fault codes that may appear along with P0491 include:

• P0492 (Secondary Air Injection System Insufficient Flow Bank 2)
• P0411 (Secondary Air Injection System Incorrect Flow)
• P0410 (Secondary Air Injection System Malfunction)
• Misfire codes like P0300-P0308

Whenever the P0491 code is set, it's important to check for these other related codes to aid in diagnosing the root issue. Repairing the P0491 issue promptly can help avoid further damage and costlier repairs down the line.

Frequently Asked Questions About P0491

What does the OBD-II code P0491 mean?

P0491 is a generic powertrain code that indicates the secondary air injection system on bank 1 (the side of the engine with cylinder #1) is experiencing insufficient airflow.

How serious is the P0491 code?

While not as severe as some codes, P0491 can potentially cause the vehicle to stall while driving, posing a safety risk. It will also likely cause the vehicle to fail emissions testing. Therefore, it’s best to diagnose and repair the issue promptly.

What are common symptoms associated with code P0491?

Symptoms may include:

• Check engine light illumination
• Rough idle or stalling during cold starts
• Increased emissions
• Reduced fuel efficiency
• Sulfur smell from exhaust

What causes a P0491 code to set?

Common causes include:

• Faulty or clogged secondary air injection pump
• Leaking or damaged vacuum hoses
• Stuck open or damaged check valves
• Wiring issues like open circuits or shorts
• Faulty electrical connections
• Failed PCM (rare)

What is the secondary air injection system and what does it do?

The secondary air injection system pumps fresh air into the exhaust stream to help reduce emissions during cold engine starts. The extra air allows for further combustion of unburned fuel before it exits the tailpipe.

What vehicles commonly have secondary air injection systems?

Secondary air injection is commonly found on Audi, BMW, Porsche, Volkswagen, and some other makes and models, especially those with V engine configurations. The specific components vary by manufacturer.

What parts make up the secondary air injection system?

Key components include:

• Secondary air injection pump
• Vacuum switching valves
• Check valves
• Vacuum hoses and lines
• Electrical wiring and relays

How do I diagnose the root cause of a P0491 code?

Diagnosis steps include:

1. Scan for codes and document freeze frame data.
2. Visually inspect wiring, hoses, and components.
3. Smoke test vacuum hoses for leaks.
4. Check electrical connections with a multimeter.
5. Test secondary air injection pump operation.
6. Inspect check valves for proper one-way airflow.
7. If no other faults are found, the PCM may be bad.

Can I drive with a P0491 code?

It’s not advisable to drive extensively with an active P0491 code. While it may not cause immediate damage, the insufficient airflow can lead to increased emissions and potential stalling. Address the issue as soon as possible.

Will a P0491 code cause my vehicle to fail an emissions test?

Yes, a stored P0491 code or even just an illuminated check engine light will likely result in a failed emissions test. The insufficient airflow in the secondary air injection system allows for increased pollutants.

How much does it cost to fix a P0491 code?

Repair costs for a P0491 code vary depending on the root cause but typically range from $400-1200 USD (€350-1100 EUR). Common repairs include:

• Secondary air injection pump: $250-600
• Vacuum hoses: $20-50
• Check valves: $40-130
• Wiring repairs: $50-200
• PCM replacement: $500-1400

Labor is usually 2-4 hours at $80-140 per hour.

Can a P0491 be caused by a vacuum leak?

Yes, vacuum leaks are a common cause of insufficient airflow in the secondary air injection system. Technicians should smoke test the vacuum lines and inspect them for cracks, kinks, or faulty connections.

How do I test the secondary air injection pump?

With the engine cold, disconnect the pressure hose from the pump. Have an assistant crank the engine and feel for strong air pressure from the pump exhaust. Little or no airflow indicates a failed pump. Also, listen for unusual noises which can indicate internal pump failure.

What is the purpose of the check valves in the secondary air injection system?

The check valves ensure air only flows in one direction – from the pump into the exhaust stream. If a check valve gets stuck open, it can allow exhaust gases to flow back into the pump, damaging it. Always inspect the check valves when replacing an air injection pump.

My secondary air injection pump failed shortly after replacement. Why?

Premature pump failure after replacement often occurs because the check valves were not inspected. Stuck open check valves allow hot exhaust gas to backflow into the new pump, causing repeat failure. Always test or replace check valves along with the pump.

Can I replace just the secondary air injection pump to fix a P0491 code?

While a faulty pump is often the culprit, it’s important to diagnose and address any other potential issues like vacuum leaks, wiring faults, or stuck check valves. Replacing just the pump may not resolve the underlying problem and can lead to repeated failures.

Are there any special steps to follow when diagnosing a P0491 code?

Most of the diagnostic tests for a P0491 code should be performed when the engine is completely cold. Some follow-up tests may require letting the engine cool down again for a few hours. Also, be aware that certain tests may temporarily trigger additional codes that need to be cleared.

What other codes may be related to P0491?

Other codes that may appear alongside P0491 include:

• P0492 (Insufficient flow on bank 2)
• P0411 (Incorrect flow)
• P0410 (System malfunction)
• Misfire codes P0300-P0308

Analyzing these related codes can provide clues to help pinpoint the root cause of the insufficient airflow.

Can I bypass or delete the secondary air injection system to prevent P0491?

While it may be tempting, it is never advisable to bypass or delete any emissions control devices, as this is illegal. Disabling the secondary air injection system will likely result in a failed emissions test and may cause other driveability issues. Always repair the system properly to maintain low vehicle emissions.

What can I do to prevent a P0491 code from reoccurring?

To keep the secondary air injection system operating properly and prevent insufficient flow issues:

• Regularly inspect and replace vacuum hoses
• Keep electrical connections clean and secure
• Address any engine issues that may cause backpressure
• Follow recommended service intervals for the air injection system
• Use OEM quality components if replacement is needed

If you suspect an issue with the secondary air injection system, have it diagnosed promptly by a professional to avoid further damage or problems. Proper maintenance and timely repairs when needed will help prevent recurring P0491 codes.