P0491: Secondary Air Injection System Insufficient Flow, Bank 1
P0491 means the engine computer detected too little fresh air flowing into the secondary air system on bank 1, the side of the engine with cylinder 1. The car usually drives normally, but the check engine light stays on and the car may fail an emissions test until it is repaired.
You can keep driving
The data lists no change in driving and no limp mode for this code. The secondary air system only works for a short time after a cold start, so the car should drive normally. The check engine light will stay on, and the car will likely not pass an emissions test until the fault is fixed.
What it means
After a cold start, a secondary air pump blows fresh air into the exhaust right behind the exhaust valves. This helps unburnt fuel burn off and warms up the catalytic converter faster. Regulations require the air flow to be monitored for each bank of cylinders. P0491 is set when the monitor finds too little air reaching bank 1. On VW and Audi engines, the computer estimates the air mass from how the front oxygen sensor reacts during the cold-start phase. An Audi bulletin describes values of 0.4 to 0.7 on both banks together as too little air, often because the air passages in the cylinder head are clogged with carbon deposits (coked). On VW and Audi V engines, each bank has its own combination valve while the pump is shared. Ford uses this code for the bank 1 secondary air monitor, while GM uses P0411 instead.
Likely causes
- Secondary air passages in the cylinder head cokedVery likely
Carbon deposits narrow the passages that carry air to the exhaust valves, so too little air gets through even when the pump runs.
- Combination valve stuck or stiffVery likely
The combination valve must open to let air in and close to protect the pump, and a stiff valve restricts the flow to bank 1.
- Vacuum supply or changeover valve leaking, valve closes too earlyPossible
If the vacuum that opens the valve leaks away, the valve shuts before the monitor has measured enough air.
- Secondary air pump weak or water in the pumpPossible
A weak or water-damaged pump cannot move enough air.
- Hose between pump and valve leaking or offLess likely
A leaking or disconnected hose lets air escape before it reaches the exhaust.
What you may notice
- Check engine light on
- No noticeable change in driving
- Secondary air readiness not set, which shows up in an emissions test
- Possibly no pump noise after a cold start
What to check, cheapest first
- 1Start the engine cold and listen for the secondary air pump running for a short time. Note whether you hear it at all. · you can do it
- 2Look at the hoses between the pump and the valves for cracks, loose ends, or disconnections. · you can do it
- 3Read the stored codes and freeze frame data, and check for related codes such as P0492, P0410, P0411, or P2440. · mechanic
- 4Check the vacuum supply and the changeover valve for leaks, since a leak can make the valve close too early. · mechanic
- 5Test the secondary air pump and the combination valve for bank 1, including checking for water in the pump and a stuck or stiff valve. · mechanic
- 6Inspect the air passages in the cylinder head for carbon buildup, especially if the same low flow shows on both banks. · mechanic
Related codes
Frequently asked questions
Can I drive with P0491?+
Yes. The data lists no driving changes and no limp mode. Have it checked soon, since the light stays on and an emissions test is likely to fail.
Can a loose gas cap cause P0491?+
No. This code is about air flow in the secondary air system, not the fuel system. The data lists causes such as clogged air passages, a stuck valve, a leaking vacuum supply, a weak pump, and a loose hose.
Will the car pass an emissions test with P0491?+
Probably not. The code is emissions relevant and turns on the check engine light, and the secondary air readiness is not set. After the repair, the car needs a drive cycle so the monitor can run again.
Written with the help of AI from the public OBDex definition (CC0). Each make may use the code differently. Updated: 2026-11-10