Fix P0491: Mechanic’s solutions that WORKED

The code P0491: Secondary Air Injection System: Bank 1: Insufficient Flow indicates an issue with the flow in the secondary air injection system for bank 1 of the engine. This system helps reduce emissions by injecting fresh air into the exhaust manifold to aid in the combustion of exhaust gases. Insufficient flow suggests a problem in this system, affecting the vehicle’s emissions and possibly its fuel efficiency.

P0491 quick overview

Meaning	P0491: Secondary Air Injection System: Bank 1: Insufficient Flow
Is it serious?	This code indicates a problem in the emission control system, which should be addressed but is typically not an immediate threat to engine performance.
Possible causes	– Faulty secondary air injection pump – Blocked or leaking air injection hoses or tubes – Malfunctioning check valves – Faulty air solenoids – Issues with the vehicle’s powertrain control module (PCM)
How to diagnose?	Inspect Air Pump: Check the secondary air injection pump for functionality and signs of water intrusion. Check Hoses and Tubes: Examine air injection hoses or tubes for blockages or leaks. Test Check Valves: Verify if the check valves are functioning correctly and not stuck. Examine Air Solenoids: Ensure air solenoids are operating and opening correctly. Evaluate PCM: Consider the health of the PCM, especially if other related codes are present. Clear Codes and Recheck: After addressing potential issues, clear the code and monitor if it reappears.

How to fix P0491 – learn from mechanics

I looked into P0491 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Audi

Vehicle Information: 2009 Audi Q5 Premium 3.2L

Issue + Repair History:

• MIL lamp on with codes P2070 and P0491.
• Initial diagnosis did not reveal any faults with the intake manifold.
• Found carbon buildup on the back of intake valves and performed walnut blasting.
• Cleaned ports with Berry-man’s Carb Cleaner.
• Tested plumbing (Y Hose) to Secondary air valves with a smoke machine.

Other OBD2 Codes + Test Results:

• P2070 – Intake Manifold Tuning Valve (Stuck Open).
• P0491 – Secondary Air Injection System Insufficient Flow (Bank 1).

Suggestions to Fix Issue:

• Check the vacuum element and flap in front of the intake manifold.
• Inspect for leaks in the vacuum line.
• Consider replacing the intake manifold if operating levers are broken.
• Perform secondary air injection basic setting and read Measure Value block for flow.
• Check Air Injection Vacuum control valve.

Final Fix:

• The air injection port on bank #1 was plugged up.
• The vehicle was taken to the dealer under a revised campaign for the P0491 Air Injection code.
• The dealer used a special water blasting tool to clean the Air Injection ports at no charge.
• Cleaning the back of the intake valves resolved the P2070 Intake code.
• The customer reported no issues and the Check Engine Light remained off after the fix.

Case #2: BMW

Vehicle Information: 1999 BMW 750iL 5.4L

Issue + Repair History:

• MIL lamp on with no driveability issues reported by the customer.
• Difficulty finding a description for the code in Alldata and other networks.

Other OBD2 Codes + Test Results:

• PO491

Suggestions to Fix Issue:

• Check secondary air injection system, including wiring, AIR solenoid, and hoses.
• Inspect hoses in front of the engine, especially those inboard of the distributor caps, for splits or damage.
• Consider using a GT-1 for accurate diagnostics, as the code might not apply to the M73 engine but to the M52TU-54 engines.

Final Fix:

• The code was cleared, and the customer decided to monitor if the code would return.
• No further codes have been reported since the initial clearing.
• When/if the code reappears, the focus will be on the secondary air injection system for potential issues.

Case #3: Audi

Vehicle Information: 2008 Audi Q7 Premium 4.2L

Issue + Repair History:

• Codes P0491 and P0492 were present.
• Replaced the secondary air injection pump due to faultiness (water found in the disconnected hose).
• Conducted multiple tests, including checking power and ground to the pump, verifying vacuum to solenoids, smoke testing the system, and checking combination valves.
• Replaced the secondary air MAP sensor, but codes returned.

Other OBD2 Codes + Test Results:

• P0491 – Secondary Air Injection System: Bank 1: Insufficient Flow.
• P0492 – Secondary Air Injection System: Bank 2: Insufficient Flow.

Suggestions to Fix Issue:

• Check the sequence of the Secondary Air Injection (SAI) system, including solenoids/valves operation.
• Ensure the solenoids open allowing air to flow into the exhaust and that the oxygen sensors read lean.
• Verify that the valves close properly and “dead head” the pump in the second part of the SAI sequence.

Final Fix:

• The issue was resolved by correcting the installation of the check valve to the solenoids, which was installed backwards, preventing vacuum to the solenoids.

Case #4: Porsche

Vehicle Information: 2004 Porsche Boxster S 3.2L

Issue + Repair History:

• The car displayed secondary air faults P0491 and P0492.
• Secondary air pump checked and found to have good airflow.
• Vacuum gauge used to test the air valve showed cycling at 15psi.
• Mechanical air valve held vacuum and plunger movement was audible.

Other OBD2 Codes + Test Results:

• P0491 – Secondary Air Below Limit.
• P0492 – (Description not provided).

Suggestions to Fix Issue:

• Check for damage or disconnections in the plastic lines on top of the engine.
• Inspect for clogging in heads from carbon and rust.
• Examine the diverter valve for proper functioning.
• Manually apply vacuum to the diverter valve and observe O2 sensor readings.

Final Fix:

• Smoke testing the exhaust revealed cracked headers.
• Replacing the catalytic converters resolved the issue.
• Used a magnehelic gauge to test for clogged head ports; the gauge showed movement during secondary air short test indicating the ports were okay.

Case #5: Chrysler

Vehicle Information: 2005 Chrysler Sebring 2.4L

Issue + Repair History:

• Code P0491 for secondary air injection problem.
• Found a 40 amp fuse blown and the electric air pump full of water.
• Replaced the fuse and reinstalled the air pump after draining and bench testing it.
• The code reappeared after a few days.
• Checked the check valve, which was not bypassing exhaust.
• Replaced the Secondary air MAP sensor.

Other OBD2 Codes + Test Results:

• P0491 – Secondary air injection malfunction.

Suggestions to Fix Issue:

• Inspect for water leakage from the air conditioning system onto the pump.
• Check and replace the AIR check valve if necessary.
• Ensure the vacuum-operated check valve functions correctly.
• Check for condensation build-up and exhaust gas backing up in the hoses.
• Consider drilling a small hole near the pump for water drainage.
• Check and possibly replace the relay for the air pump.

Final Fix:

• The check valve was found stuck closed and was replaced.
• The system worked properly after the new valve installation but the code still appeared.
• Identified the MAF sensor as faulty through voltage testing.
• The customer couldn’t afford to replace the MAF sensor, but it was identified as the likely source of the problem.

Advice	Brief Description of Diagnostic Step
Check Secondary Air Pump	Inspect the air pump for functionality, water intrusion, and proper airflow.
Inspect Check Valve	Examine the check valve for proper operation and ensure it’s not stuck or blocked.
Test for Vacuum and Air Flow	Verify if the vacuum is reaching the solenoids and if air flow is adequate in the system.
Smoke Test for Leaks	Conduct a smoke test to identify any leaks in the secondary air injection system.
Inspect Electrical Components	Check related fuses, relays, and wiring for damage or malfunction.
Replace Faulty Components	Consider replacing faulty components such as the MAP sensor or air pump if diagnosed.
Clear Codes and Monitor	After repairs, clear the codes and monitor the system to ensure the issue is resolved.
Check for Condensation or Water Damage	Look for signs of condensation or water ingress that could affect system components.