P0491 code meaning
AIR System Insufficient Flow Bank 1
Powertrain system Generic code P04xx codes Check engine light
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. 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.
P0491 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 4 hours
- Clears after repairAfter a drive cycle
Symptoms of P0491
- Check engine light on
- No change in driving
- Secondary air readiness not set (emissions test)
- Possibly no pump noise after a cold start
Common causes of P0491
- Most likelySecondary air passages in the cylinder head coked
- Most likelyCombination valve stuck or stiff
- PossibleVacuum supply or changeover valve leaking, valve closes too early
- PossibleSecondary air pump weak or water in the pump
- Less likelyHose between pump and valve leaking or off
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Secondary air pumpAir combination valve bank1Secondary air passagesFront oxygen sensor
What each part of P0491 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| 91 | The exact fault in that area. |
What to do next
- Note P0491 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0491
What does P0491 mean?
P0491 means "AIR System Insufficient Flow Bank 1". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P0491?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0491 fail an emissions test?
Yes, it can. P0491 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0491 to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will P0491 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.