P0491 on Audi
Secondary Air Injection System Insufficient Flow (Bank 1)
P0491 on Audi vehicles indicates secondary air injection system insufficient flow (bank 1). Au7o has documented this code across 1 Audi model — most commonly on A8. This code means the PCM determined that the secondary air injection system is delivering insufficient airflow into the exhaust on Bank 1. This system pushes fresh air into the exhaust during cold starts to help the catalytic converter warm up quickly and lower emissions. Insufficient flow means the system isn't doing its job, usually due to a blockage, leak, or weak air pump. It typically triggers the check engine light and affects cold-start emissions rather than everyday driveability. Typical repair costs on Audi range from $80 to $1,400, depending on the specific model and root cause.
Common Causes of P0491
- •Clogged or restricted secondary air injection passages
- •Failing secondary air injection pump
- •Stuck or leaking air switching/check valve
- •Vacuum or hose leak in the air injection plumbing
- •Carbon buildup blocking air delivery ports
- •Electrical fault in the pump or valve circuit
P0491 on Audi by Model
Audi A8(1 issue)
- Secondary Air Injection P0410/P0411 — Cracked Vacuum Lines, Stuck Combi Valves, Carbon-Clogged Head Ports1997-2003
The 4.2 V8 in the D2 A8 (both the early 32-valve ABZ and the later 40-valve AKB/AUX/AYS/BFM) runs a vacuum-operated secondary air injection system with one combi valve per cylinder head, fed by small-diameter hard plastic vacuum lines that become brittle with heat and crack. Both banks have their own combi valve, and it is usually the hard vacuum line that goes to them that breaks and causes a vacuum leak and inoperable SAI. Owners report replacing vacuum hoses and the SAI relay only to have the CEL return, and then having to look at replacing the SAI pump (SAIP), the solenoid on top of the engine, and replacing/cleaning out the combi valves aft of each cylinder head, which involves carbon buildup in the cylinder head ports. Symptom is a check-engine light shortly after a cold start with P0410 (or banked variants P0491/P0492).
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What does P0491 mean on Audi?▼
P0491 stands for "Secondary Air Injection System Insufficient Flow (Bank 1)." This code means the PCM determined that the secondary air injection system is delivering insufficient airflow into the exhaust on Bank 1. This system pushes fresh air into the exhaust during cold starts to help the catalytic converter warm up quickly and lower emissions. Insufficient flow means the system isn't doing its job, usually due to a blockage, leak, or weak air pump. It typically triggers the check engine light and affects cold-start emissions rather than everyday driveability. On Audi specifically, this code is documented across 1 model.
What causes P0491 on Audi vehicles?▼
Common causes on Audi: Clogged or restricted secondary air injection passages, Failing secondary air injection pump, Stuck or leaking air switching/check valve, Vacuum or hose leak in the air injection plumbing, Carbon buildup blocking air delivery ports. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0491 on a Audi?▼
Repair costs on Audi range from $80 to $1,400, depending on the specific model and root cause.
Which Audi models have P0491 documented?▼
Au7o has documented P0491 on 1 Audi model: A8.