P0491 VW/Audi Fault Code
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P0491 VW/Audi Fault Code: Diagnosis & Repair

Chandler Stark

Meet Trey

Trey is an automotive enthusiast and has a huge passion for Volkswagen and Audi vehicles of all kinds. His enthusiasm started with the MK5 GTI, and he has massively expanded his knowledge over the years. When Trey is not delivering high-quality and in-depth content, we can usually find him working in his garage on his modified Genesis coupe. Trey created VW Tuning several years ago, and he is the primary visionary behind the content.

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P0491 VW/Audi Fault Code: Secondary Air Injection System Insufficient Flow Bank 1

When you receive a P0491 code on your Volkswagen or Audi, it will read out “Secondary Air Injection System Insufficient Flow Bank 1”. This engine code can cause some confusion when it comes to diagnosis. The secondary air injection system includes a secondary air pump, secondary air pressure sensor, a combination (kombi) valve, a control relay, and numerous hoses. It seems that this code is most popular on VW/Audi 2.5L engines. The secondary air injection system injects air into the exhaust to lower emissions. On a cold start, the secondary air pump should kick on to break down unburned fuel and heat up the catalytic converter. 

Can you still drive with a P0491 VW/Audi Fault Code?

Yes, you can drive on this code normally but can be dangerous it is causing engine stalls while the vehicle is in motion. If you are in a state that takes emissions seriously on inspections, this code may cause the vehicle to fail emissions tests. We would suggest addressing it as soon as possible however because the engine is not optimally functioning. We would advise not ignoring this for more than a couple of weeks because the longer it goes, the more likely more issues will start to arise.

P0491 VW/Audi Symptoms

  • Check Engine Light (CEL) or Malfunction Indicator Light (MIL) illuminating
  • Whistling or hissing sound coming from the engine
  • Hesitation on acceleration
  • Engine stalls
  • Engine backfiring
  • Rough idle

Causes of P0491 Engine Code for Volkswagen’s or Audi’s

  • Faulty secondary air pressure sensor
  • Damaged or loose wires to the air pressure sensor
  • Secondary air injection relay failure
  • Faulty secondary air fuse failure
  • Secondary air injection pump failure
  • Secondary air hose leak
  • Combination (Kombi) valve failure

How to Repair a Volkswagen or Audi with a P0491 Engine Code

As we have stated above, a P0491 VW/Audi engine code can be quite difficult and expensive to diagnose. Therefore we have laid out causes that are common to pop this code. The last thing you want to do is order a secondary air injection pump because you are getting a P0491. This is not always the case. So go step-by-step with this guide to assist in diagnosis. Before getting into the guides below, we want to preface that all of the replacement parts and DIYs are for a Volkswagen/Audi 2.5L engine because it is the most popular VW/Audi engine that this code pops up on.

Check or Replace Secondary Air Pressure Sensor

The secondary air pressure sensor is found on the secondary air pump and its purpose is to monitor airflow from the secondary air injection pump. Surprisingly, it is a little more expensive than we thought to replace the sensor, but a shop will charge around $350 to replace it.

Buy Here: Volkswagen/Audi 2.5L Secondary Air Injection Sensor Kit
DIY Difficulty:
Intermediate

Check or Replace Secondary Air Injection Relay

This relay can be numbered many different things depending on if it is an Audi or Volkswagen, different engines, different generations, and more. We know of it being the 373 relay (Passat) or J299 (2.5L) relay, but there are different numbers out there. The best thing to do is to look in your car’s manual for relay and fuse layouts. You’ll want to look for the “Secondary Air Injection (AIR) Pump Relay”. It is located inside the engine compartment on the left side in a covered black box.

DIY Difficulty: Easy

Check or Replace Secondary Air Injection Fuse

A blown secondary air injection fuse could cause the pump to not function, which would throw the P0491 VW/Audi code. You can find the secondary air injection fuse at any auto store. It is a $0 amp fuse that is located in the fuse box in the engine’s cabin. To find the exact location, you should be able to find this in your vehicle’s manual.

DIY Difficulty: Easy

Replace the Secondary Air Injection Pump

A secondary air injection pump, often referred to as SAI pump or smog pump, lowers the engine’s emissions coming out of the tailpipe. The pump can get water into if a check valve is not functioning properly, causing the water to freeze in the air pump. When this occurs, the secondary air injection pump will start to fail or even fail immediately. If ignored, a secondary air pump can cause fouled spark plugs.

Buy Here: Volkswagen/Audi 2.5L Secondary Air Injection Pump
DIY Difficulty:
Intermediate

Check for Secondary Air Injection Hose or Pipe Leaks

The secondary air injection pipe connects the secondary air pump to the combination valve. These pipes seem to be a little brittle out of the factory which leads to premature failure. If air leaks occur, the P0491 VW/Audi fault code will pop up and the engine will run rough.

Buy Here: Volkswagen/Audi 2.5L Secondary Air Injection Hose
DIY Difficulty:
Intermediate

Check or Replace the Combination (Kombi) Valve

Lastly, the combination, Kombi, valve controls the secondary air injection going to the exhaust system. If the valve is stuck open it could lead to moisture going into the air pump and ruining that component as well.

Buy Here: Volkswagen/Audi 2.0T Secondary Air Injection Kombi Valve
DIY Difficulty:
Intermediate

P0491 Volkswagen/Audi Conclusion

We hope the information provided above has alleviated the P0491 VW/Audi engine code. When the code pops up, it isn’t a very severe code, so it can be driven off. Obviously, we suggest trying to diagnose this on your own with the steps above or taking it in for service within a few weeks of receiving the code. If this guide helped you alleviate the engine code, let us know what was wrong and how you fixed it in the comments below to assist future readers. Lastly, if you need assistance in finding guides or replacement parts for a different engine, let us know in the comments below and we will assist in any way we can.

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One Comment

  1. Hello,
    My name is Sam and I have a 2015 MK7 GTI autobahn Canadian version with 106000 km. Recently my check engine light came on and OBDII gave me a P0491 code. This fault code/check engine light and abnormally loud low pitch grinding/growling noise only come on for 30 to 90 seconds and only happen with an overnight cold start. The car idles and runs very smoothly during cold start and at all speeds. I disconnect the secondary injection pump and connected it to a 12-volt battery and the sound seems normal but I did not do a vacuum test. Could you please give me some advice on how to approach this task? Thank you!

    Regards,
    Sam

    PS I erased the fault code and that is why I know the check engine light only come on at cold start.

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