P0491 - Secondary Air Injection System Insufficient Flow (Bank 1)
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0491 occurs when the Engine Control Unit (ECU) detects that the Secondary Air Injection (SAI) system on Bank 1 is not flowing enough air. The system injects fresh air into the exhaust manifold during cold starts to oxidize unburned hydrocarbons and speed up catalytic converter light-off. The ECU monitors air flow by observing the Pre-Catalyst Oxygen Sensor (O2). When the pump is active, the O2 sensor voltage should drop significantly due to high oxygen content. If the voltage does not drop below a specific threshold (typically <0.2V) within a set time, P0491 is stored. This system is inactive during normal warm operation but vital for cold-start emission compliance.
Possible Causes
Failed Secondary Air Injection pump (internal motor failure or water intrusion)
Stuck or faulty one-way check valve (Kombi-valve)
Cracked, disconnected, or leaking vacuum lines or plastic air hoses
Blown fuse or faulty SAI pump relay
Carbon buildup clogging the air passages in the cylinder head
Symptoms
Recommended Solution
Workflow
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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