Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
Your car has a system that helps reduce pollution by injecting air into the engine. This code means that the system isn't working well on one side of the engine.
The P0491 code indicates that the Secondary Air Injection (AIR) system is not providing enough air flow to the exhaust on bank 1 of the engine.
The Secondary Air Injection system is designed to reduce emissions by injecting fresh air into the exhaust stream during cold engine start-up. This helps burn off excess hydrocarbons. The P0491 code is triggered when the Engine Control Module (ECM) detects that the air flow is insufficient in bank 1, which could be due to blockages, leaks, or component failures.
• Check Engine Light is on
• Failed emissions test
• Rough idle during cold start
• Faulty air pump
• Blocked or leaking air injection hoses
• Faulty air injection check valve
Secondary Air Injection: A system that injects air into the exhaust to help reduce emissions.
Bank 1: The side of the engine where cylinder number 1 is located.
• Check for any leaks or blockages in the air injection hoses.
• Test the air pump for proper operation.
• Inspect the air injection check valve for proper function.
• Replace faulty air pump.
• Clear blockages in air injection hoses.
• Replace faulty air injection check valve.
• OBD-II scanner
• Vacuum gauge
• Multimeter
$150 - $500
Moderate
• BMW 3 Series
• Audi A4
• Volkswagen Jetta
• Regularly inspect air injection hoses for wear.
• Ensure air pump is functioning properly.
• Perform regular emissions checks.
Engine Performance
• Address this issue promptly to avoid failing emissions tests.
• Consult a professional mechanic if unsure about repairs.
Check with your vehicle manufacturer for emissions-related warranty coverage.
The P0491 code has been a common issue in vehicles with secondary air injection systems, especially in models from the early 2000s.
Moderate – While the car may run, it can lead to increased emissions and potential engine performance issues.
Driving with this code may lead to higher emissions, which could be against local environmental regulations.
• Cold engine start
• Ambient temperature below 50°F
The Secondary Air Injection system components are typically located near the front of the engine bay, including the air pump and associated hoses.
Q: Can I drive with a P0491 code?
A: Yes, but it's not recommended as it may lead to increased emissions and potential engine performance issues.
Q: How can I pass an emissions test with a P0491 code?
A: You will need to fix the issue causing the code, such as repairing or replacing faulty components in the AIR system.
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Fitment Required · Vehicle-fitment part
Relevant for airflow, fuel-trim, and intake-metering faults after confirming the sensor is actually at fault.
Airflow and fuel-trim codes can involve the mass air flow sensor after proper diagnosis.
Verify the sensor design and fitment for your exact vehicle.
View options on AmazonThese parts may be related to this fault category, but a code by itself does not confirm exact fitment or guarantee that replacing a part will solve the problem.