P0491 Secondary air injection system is not moving enough air (bank 1)

Secondary air injection system insufficient flow, bank 1

Severity not verified — if the car runs badly, have it inspected

The sources read describe the fault but do not rate driving risk. It is an emissions fault that sets at cold start.

Source: how we rate severity

P0491 in short

P0491 means the computer saw too little air from the bank 1 secondary air system, which blows fresh air into the exhaust at cold start. Carbon-blocked ports, a weak pump or valve, a hose leak or a fuse are typical causes. Check the fuse first; severity is not verified.

For a short time after a cold start, the secondary air pump pushes air into the exhaust so leftover fuel burns and the catalytic converter warms faster. The computer checks that enough air arrives. If it does not, it stores P0491.

Anything between the pump and the exhaust can cut the flow: a fuse or relay, a leaking hose, a stuck combi valve, or ports that have filled with carbon. Audi bulletins single out the ports in the cylinder head, and the Audi cars in them set the bank 2 code P0492 too.

A few Volkswagens have a software bulletin because the computer was setting the code in error.

Most likely causes

  1. Carbon clogging the secondary air ports in the cylinder head (Audi V6 and V8 models)

    Supported by vehicle data
    Why it ranks here
    Audi bulletin A011383 says the ports can accumulate carbon and restrict flow, on cars with P0491 and P0492 together. Ross-Tech lists the same carbon blockage and says cleaning the ports is the fix.
    What would change this
    Rises if both bank codes are stored, the pump runs normally, and the air valve opens. Falls if the pump or valve fails its own test.

  2. Failed air pump, combi valve or solenoid

    Common general cause
    Why it ranks here
    Ross-Tech lists defective solenoids, a failed combi valve and the pump among the causes; TAT lists the pump and control valve.
    What would change this
    Rises if the pump does not run or the valve does not open when commanded. Falls if both operate and air flows.
    How to test the secondary air pump

  3. Leaking hose or pipe, blown fuse or bad relay

    Common general cause
    Why it ranks here
    Ross-Tech lists leaking hoses and piping between the pump and valve and a faulty fuse or relay as causes.
    What would change this
    Rises if a fuse is open, a hose is cracked or air escapes before the valve. Falls if the plumbing and fuse are intact.
    How to test the vacuum hoses and intake leaks

  4. Engine software that sets the code in error (2013-2015 Jetta, Beetle, 2014-2015 Passat)

    Supported by vehicle data
    Why it ranks here
    VW bulletin V011514 says the current ECM software causes erroneous faults for P0491 and/or P2440 and supplies an update. It is limited to those models.
    What would change this
    Rises if there is no hardware fault and the VIN is covered. Falls if a test shows low airflow.
    How to test the engine control module (ecm / pcm)

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Check the secondary air pump fuse and relay, and the pump connector.

    Low risk

  2. With a scan tool that supports it, run the secondary air output test and listen to the pump while watching for air at the valve.

    Low riskTool: bidirectional scan tool

  3. On an Audi or Volkswagen, ask a dealer whether your VIN has a bulletin: carbon in the cylinder head ports, or a software update.

    Best left to a shop

    Warning:Cleaning the ports needs the intake or cylinder head parts removed; leave it to a shop.

Symptoms

  • Check engine light, often with no change in driving
  • A loud whirring or no whirring from the pump at cold start

How to fix

  1. Find the secondary air pump fuse in the fuse box and check that it is not blown.

Parts and cost context

  • Secondary air pump:Confirm the pump and valve fail their tests before replacing them. Labour varies; get a quote.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Audi

  • 2009–2012 · A6

    Supported by vehicle data

    Audi A011383 and A261215 cover P0491 and P0492 set together, with carbon in the secondary air ports. Whether they are still current is not verified.

  • 2009–2012 · Q5

    Supported by vehicle data

    Same Audi bulletins apply to the Q5. Whether they are still current is not verified.

  • 2009–2012 · A4

    Supported by vehicle data

    Same Audi bulletins apply to the A4. Whether they are still current is not verified.

  • 2011–2012 · A8

    Supported by vehicle data

    Audi has a repair authorization for P0491 or P0492 on the 4.2 FSI A8. Whether it is still current is not verified.

  • A011383Status unverified

    MIL on, P0491 and P0492 stored together

    2009–2012a4, a6, q5, q7 · issued · NHTSA 10123886

    Secondary air ports in the cylinder head can accumulate carbon and restrict flow. The summary does not give the repair steps.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • A261215Status unverified

    P0491 and P0492 stored at the same time

    2009–2012a6, a4, q5, s6 · issued · NHTSA 10122864

    Supersedes an earlier 2012 bulletin; mileage over 20,000. The summary does not give the repair steps.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • 0112652030230Status unverified

    A8 4.2 FSI with P0491 or P0492

    2011–2012a8 · issued · NHTSA 10250421

    Repair authorization bulletin; the summary does not give the repair.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Volkswagen

  • 2013–2015 · Jetta

    Supported by vehicle data

    VW V011514 updates the ECM for P0491 and/or P2440. The bulletin also lists Beetle and Passat. Whether it is still current is not verified.

  • V011514Status unverified

    Update programming for P0491 and/or P2440

    2013–2015jetta, beetle, passat · issued · NHTSA 10122989

    Current ECM software causes erroneous faults; this bulletin has the codes needed to update it.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P2440Secondary air switching valve stuck open (bank 1)

    Same system Supported by vehicle data

    Volkswagen bulletin V011514 covers P0491 and P2440 together, because the computer may set them in error.

All Secondary air injection codes

Sources and review

  1. P0491 - Secondary Air Injection System: Bank 1: Insufficient Flow — Ross-Tech Wiki
  2. Code view: P0491 secondary air injection system low flow bank one — TAT scan data code list
  3. Audi A011383: MIL on, P0491 and P0492 stored together, carbon in secondary air ports — Audi of America / NHTSA ,
  4. Audi A261215: P0491 and P0492 stored at the same time (supersedes 26 12-10) — Audi of America / NHTSA ,
  5. Volkswagen V011514: update programming for P0491 and/or P2440 — Volkswagen of America / NHTSA ,
  6. Audi repair authorization: A8 4.2 FSI with P0491 or P0492 — Audi of America / NHTSA ,
Last reviewed
Written by
Saurabh K.
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