P0491 on Audi: How to Diagnose & Fix
Secondary Air Injection System Insufficient Flow (Bank 1)
3 Audi models · 3 documented issues · $80-$2,000 repair
P0491 on Audi by model: symptoms, diagnosis and fix
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).
Common Symptoms
- Check engine light after cold start
- Audible hissing from engine bay at cold start
- Rough idle for first 30-90 seconds after cold start
- Failed emissions/smog test
- Pump noise (whining) on cold start
How to Fix
Diagnose by starting the engine cold so the SAI system runs and listening for the vacuum leak, then looking around for the crack. Sticky/partly opening combi-valve or slow running pump are all candidates — you can spin the pumps up using VCDS output tests to verify pump current draw. Replace brittle vacuum hose with silicone, test combi valves with a hand vacuum pump (they should hold vacuum and audibly click open), and if codes persist with new vacuum/valves the SAI ports in the head will need to be carbon-cleaned. The pump itself (smog pump) typically fails when a leaking combi valve allows exhaust moisture to back-feed and corrode the impeller.
Sources for this issue (1)
Audi TSB 2033001/18 applies to 2011-2016 Q7 3.0 TFSI vehicles with more than 15,000 miles when the malfunction indicator lamp is on and P0491 and P0492 are stored together. Audi identifies carbon restriction in the cylinder-head secondary-air ports. This is not an intake-valve diagnosis and the bulletin does not establish universal valve damage, a walnut-blasting maintenance interval or a need for engine replacement.
Common Symptoms
- Malfunction indicator lamp on
How to Fix
Have an Audi-trained technician confirm that P0491 and P0492 are present together, test the secondary-air pump and hose routing, and measure secondary-air flow using the bulletin procedure. If the measured flow and checks match the bulletin, the cylinder-head secondary-air ports are cleaned with Audi tools and training. Do not buy a PCV assembly, catch can or intake-cleaning product from this card.
Sources for this issue (1)
The secondary air injection (SAI) system pumps air into the exhaust ports on cold start to light off the catalysts, and it is a chronic check-engine-light source on these cars. Each bank has its own combi valve. Carbon causes the combi valve to stick open, which lets hot, moist exhaust gas flow backward and condense inside the pump housing, rusting the motor bearings - the source of the loud jet-engine or whining noise on cold start. A separate and commonly misdiagnosed variant produces the same P0491/P0492 insufficient-flow codes with NO pump noise at all: the air passages inside the cylinder heads are carboned shut, and replacing the pump will not fix it. Brittle hard vacuum lines to the combi valves are a third cause.
Common Symptoms
- Loud whining, jet-engine or grinding noise from the engine bay for the first ~30-90 seconds after a cold start
- Check engine light with secondary air flow codes
- Failed emissions / readiness monitor not setting
- Noise absent but codes still return after clearing
How to Fix
Diagnose before buying parts. Loud pump noise plus codes points to a water-damaged pump - and if the pump is replaced, the combi valve on that bank MUST be replaced too or the new pump will rust out the same way. Codes with a quiet, functioning pump point to clogged head passages, which require removing the combi valves and mechanically clearing the ports. Also inspect and replace the hard vacuum lines and the changeover solenoid, which are cheap and commonly cracked. Failing SAI will fail an emissions test even though driveability is normal.
Sources for this issue (3)
P0491 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0491 mean on a 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 it is documented across 3 models: A8, Q7, S4.
What are the most common causes of P0491?
The most common causes of P0491 are: 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. The specific cause varies by vehicle.
How much does it cost to fix P0491 on a Audi?
Published repair estimates on Audi range from $80 to $2,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0491 on a different make?
View P0491 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.