Fault Code P0491

[Russtty]

Would anyone be able to give me a bit more info on Fault Code P0491? The CEL came on when I was driving home from work. I can't say if it came on when I started the car. I noticed it after about 2 mins of driving. Doing some searching, it would appear to be a code for the secondary air system. Any info on typical causes and where I should have a look would be appreciated! I did go over the engine bay to see if there were any hoses that were obviously off or out of place without any luck. I do admit I don't know specifically which hoses I should be looking at though.

Thanks

Steve

[rktskicar]

Do you have a VAG (VCDS)? If so, reset the code and see if it comes back.

Bruce

[Russtty]

I reset the code this morning when starting the car. It did not come back on the drive. I'm going to go out at lunch and give it another start to see if the code comes back when cold. What function does the Secondary Air system serve? Reading other threads I got the impression that it is for cold starts only to clear out the cat. My dealer is saying something about possible carbon buildup on ports. The car just had the 40,000km service performed on Monday. :/

[rune]

P0491
16875

Corrective action
- Check fuse for Secondary AIR Pump

[Russtty]

I cleared the fault and it hasn't come back yet. Which bank is that fuse in? I didn't see one specifically labelled for the air pump. Reading other posts with regards to older models, the fuse is a 40A, but the only 40A I saw in this car was for the A/C

Thanks for the input!

Steve

[rktskicar]

I cleared the fault and it hasn't come back yet. Which bank is that fuse in? I didn't see one specifically labelled for the air pump. Reading other posts with regards to older models, the fuse is a 40A, but the only 40A I saw in this car was for the A/C

Thanks for the input!

Steve

If the code does not come back, then don't worry about it. could be some residual code from what they did at your service. I have had two random codes show up over the last 1.5 years, that I reset and they didn't come back. For me, if the code comes back after a reset, then something to look into. If I had a nickel for all the codes I had on my stage 3 B5 S4, I would be a rich man. ;)

Bruce

[nbosco]

My car threw this code (~30k mi). P0491: Secondary Air B1 Insufficient Flow. I took it to the dealer and they did say it was an intermittent cold start issue. They first replaced the "change over valve solenoid" but I guess that didn't fix it, so they then replaced the "secondary air change over valve" and that has stopped the code.

I'm not an engine guy, so I'd be real interested to hear if anyone has any insights into this issue. Is this a sign of bigger problems to come?

[mitko]

My car threw this code (~30k mi). P0491: Secondary Air B1 Insufficient Flow. I took it to the dealer and they did say it was an intermittent cold start issue. They first replaced the "change over valve solenoid" but I guess that didn't fix it, so they then replaced the "secondary air change over valve" and that has stopped the code.

I'm not an engine guy, so I'd be real interested to hear if anyone has any insights into this issue. Is this a sign of bigger problems to come?

I had that same code come up a few times this winter after starting the car on a very cold morning. It would clear by itself in a day or so, and pop back up again when it got really cold in the morning. Took it to the dealer and they said it was due to carbon buildup in the secondary air injection system. They did a carbon cleaning under warranty and it hasn't shown up again since.

[nbosco]

I had that same code come up a few times this winter after starting the car on a very cold morning. It would clear by itself in a day or so, and pop back up again when it got really cold in the morning. Took it to the dealer and they said it was due to carbon buildup in the secondary air injection system. They did a carbon cleaning under warranty and it hasn't shown up again since.

Thanks, that's the kind of info I was looking for. If it pops up again I'll push for the "warranty carbon cleaning".

[integroid]

I thought this code was what usually came on when you have carbon build up?

[B8 S FOR]

I had that same code come up a few times this winter after starting the car on a very cold morning. It would clear by itself in a day or so, and pop back up again when it got really cold in the morning. Took it to the dealer and they said it was due to carbon buildup in the secondary air injection system. They did a carbon cleaning under warranty and it hasn't shown up again since.

I had the same code for Carbon Build Up in the Secondary Air system. 42,500 miles mine poped on. Covered under the TSB $1,200 job and 3 days later. Pretty big job. Front bumper needs to come off.

I posted some same comments related to thid in this thread...which was different from the original poster.

http://www.audizine.com/forum/showth...-after-60k-Pic

[nbosco]

Any insights into why my dealer choose to replace valves and not clean the "build up"?

[SwankPeRFection]

Any insights into why my dealer choose to replace valves and not clean the "build up"?

Because more money from Audi.

[nbosco]

Questions:
- I'm stock now but have been considering an ECU tune since I'm close to the end of my warranty. Would being flagged TD1 following my warranty period also void the special carbon buildup extended warranty?

-If that's the case, what's the out of pocket cost of a carbon cleaning?

[Ryan_qttro]

Secondary Air pumps air into the exhaust side of the head to heat the catalytic converter up quicker emissions. The 3.0T and 3.2 V6's have an issue with the secondary air ports in the head clogging. So we take the front end off and pressure wash the ports out. Pretty fun and messy.

[B8 S FOR]

Questions:
- I'm stock now but have been considering an ECU tune since I'm close to the end of my warranty. Would being flagged TD1 following my warranty period also void the special carbon buildup extended warranty?

-If that's the case, what's the out of pocket cost of a carbon cleaning?

My dealer quoted me $1,200 for the job. $0 cost to me fell under TSB. Car is now extended to 120K

Here is what was stated on my repair invoice:

-Coolant Pipe remove & install
-Remove Power Steering Unit
-Lock Carrier Loosen + Fasten
-Power Wash Secondary Ports
-2 06E-121-119-E Washer
-2 N-011-907-8 Cover
-1 06E-121-139- H Gasket
-1 06E-121-119-C Washer
-1 G-013-A8J-1G Cool Add

[P Rock]

I just had this same code pop up last night. My car had the 35k service done last fall which included carbon cleanup. I'll try resetting the code and hope it doesn't come back.

[audistealth]

I just had this same code pop up last night. My car had the 35k service done last fall which included carbon cleanup. I'll try resetting the code and hope it doesn't come back.

Don't reset the code! Take it in to your dealership and tell them to perform the extended warranty service for carbon buildup. Deleting the code doesn't actually help you at all, it saves Audi money.

[P Rock]

Don't reset the code! Take it in to your dealership and tell them to perform the extended warranty service for carbon buildup. Deleting the code doesn't actually help you at all, it saves Audi money.

I'm out of warranty though :-/

[SwankPeRFection]

I'm out of warranty though :-/

Audi sent a letter to everyone with an S4 that extended their warranty on carbon cleaning of secondary air ducts should it happen. This means you're covered if that's the cause.

[P Rock]

Audi sent a letter to everyone with an S4 that extended their warranty on carbon cleaning of secondary air ducts should it happen. This means you're covered if that's the cause.

Anyone have details of said letter?

[moose_head]

http://www.audizine.com/forum/showth...carbon-buildup