What P0491 means
After a cold start the secondary air pump blows fresh air behind the exhaust valves so that unburnt fuel burns off and the catalyst heats up faster; CARB requires the air flow to be monitored per bank.
How the engine computer detects it
P0491 means too little air reaches bank 1 (the side with cylinder 1). VW and Audi calculate the injected air mass per bank from the front oxygen sensor's reaction during the cold-start phase; an Audi bulletin names values of 0.4–0.7 on both banks as too little together, often from coked air passages in the cylinder head. On VW/Audi V engines each bank has its own combination valve with a shared pump. Ford assigns the code to the bank 1 secondary air monitor; GM uses P0411 instead.
Written from public technical sources and not yet checked by a mechanic. How we write these pages
Symptoms
- Check engine light on
- No change in driving
- Secondary air readiness not set (emission test)
- Possibly no pump noise after a cold start
Common causes
- Secondary air passages in the cylinder head coked Most likely
- Combination valve stuck or stiff Most likely
- Vacuum supply or changeover valve leaking, valve closes too early Possible
- Secondary air pump weak or water in the pump Possible
- Hose between pump and valve leaking or off Less likely
How to diagnose P0491
- Read every stored code with an OBD-II scanner. If misfire or fuel trim codes are there too, fix those first: they often cause this one.
- Inspect the parts involved: secondary air pump, air combination valve bank1, secondary air passages, front oxygen sensor.
- Work through the causes above from most to least likely, cheapest checks first.
- Clear the code and drive a full drive cycle to confirm the repair held.
Reading the code
- P
- Powertrain
- 0
- Generic (SAE) code
- 4
- Auxiliary emission controls
- 91
- Specific fault
Questions drivers ask
Can I drive with code P0491?
Usually yes for a short time, if the car drives normally, the temperature gauge reads normal and the check engine light is steady. If the light flashes, the engine loses power or overheats, or other warning lights come on, pull over and have it checked.
Will my car pass an emissions inspection with P0491?
Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.
Will P0491 clear by itself?
The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.
This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.