Powertrain, generic · Auxiliary emission controls

P0491 AIR System Insufficient Flow Bank 1

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Secondary air passages in the cylinder head coked
Combination valve stuck or stiff
Vacuum supply or changeover valve leaking, valve closes too early
Secondary air pump weak or water in the pump
Hose between pump and valve leaking or off

Symptoms

  • Check engine light on
  • No change in driving
  • Secondary air readiness not set (emission test)
  • Possibly no pump noise after a cold start

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–900
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

secondary air pumpair combination valve bank1secondary air passagesfront oxygen sensor

Related codes

P0492P0410P0411P2440P0412P0418

References: static.nhtsa.gov, static.nhtsa.gov, static.nhtsa.gov, newtonnet.co.uk.