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My troubleshooting and repair of codes P0491 & P0492.

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289K views 76 replies 37 participants last post by  petr4271  
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#1 · (Edited)
If you are planning to remove your valve cover one day (for VCG or VANOS seals), you might want to read this just in case.

The following day after I was done with my VANOS seals kit installation, the SES light illuminated and the OBD 2 scanner informed me of 2 codes: P0491 and P0492. Great! The VANOS codes (P0014 and P0015) are gone and the secondary air control system codes are now registered. I'll explain later why it's not a coincidence. But at first I did not suspect it was related to my VANOS job. I went online and started reading on what the secondary air control system consists of. Air pump (not shown here), check valve (#1), solenoid vacuum control switch (#5), another little non-return valve (#6) and lots of vacuum line (#3,4,7) to connect these components as well as relays and fuses (not shown here).

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Nice! How do I know which component is bad and causes the codes? Searched and found this very helpful document that describes the diagnostic procedures for every unit of the E46 secondary air control system. See pages 6 and 7 of this newsletter pdf:

https://www.bavauto.com/newsletter/2008_n408_newsletter.pdf

What follows is the abbreviated version of the diagnostic procedure that pertained to my specific case. Refer to the above-mentioned link for the complete protocol.

Step # 1: rule out the pump. Easy, mine works like a beast. Step # 2: rule out the check valve. Well, it does not seem to open...

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Need to do the vacuum test. After a short trip to AutoZone I'm a happy owner of a MityVac hand vacuum pump. Now we'll see if the valve works or not. After an hour of trying to apply vacuum to the check valve, I realized that the MityVac vacuum pump the good people at AutoZone had sold me was defective: the vacuum release nipple was jammed. After fixing the vacuum pump I successfully applied vacuum to the valve, started the car and the valve opened, after releasing the vacuum it closed. So, the valve works just fine.

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It's good news and bad news at the same time. The good news is that the 2 most expensive components are ruled out as perfectly working and the rest of the units are relatively cheap. The bad news is the fact that the rest of the components are located somewhere behind and under the manifold and I do not have a clear idea of how to reach them. So, to start with I decided to check the vacuum line going from the check valve to the solenoid vacuum control switch. The line runs around the valve cover and behind the manifold. You need to remove the cabin filter assembly and valve cover and fuel rail trim covers. And lo and behold! The vacuum line is disconnected at the little adapter that connects 2 hoses together, not even torn but just disconnected.

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What happened is upon the reinstallation of the valve cover after the VANOS job I was moving the bundles of O2 sensor wires out of the way and inadvertently tugged at the hose disconnecting it. Did not even notice it at that time.

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So that was an easy fix, the codes are gone. The moral of the story? Be careful with that vacuum hose back there when you remove and reinstall the valve cover. The hose also becomes brittle and brakes at the check valve but it's much easier to notice it there. So, although this all could have been avoided had I been more careful, it was a good experience and a chance to read up on how the secondary air control system works and it's diagnostic procedures.
 
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#5 ·
mujjuman said:
That was a great read! and something to keep in mind wen doing this kind of work. thanks for sharing
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G. P. Burdell said:
Well done! :thumbup:
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Thank you. These codes (P0491 & P0492) are pretty common and I hope this info will help people to DIY this repair. The diagnostic instructions in the bavauto newsletter are excellent. Hopefully there will be no need to take the car to the dealership just to get overcharged for...a piece of vacuum hose.
 
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#8 ·
I could not edit my original post so I'm posting some additional information that I have collected here.

Paragraphs 6,7,8 and 9 of the bavauto secondary air system troubleshooting guide (see original post) discuss diagnosing system components located below and behind the manifold in a hard to reach place. Bimmerfest member Genuity shared the pictures of these components (electrical vacuum valve and non-return valve) that he took as a part of his car overhaul project. I appreciate his contribution and insert these pictures as a part of the origilal write-up.

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Good luck! :)
 
M
#14 ·
After replacing the valve cover gasket, my car looks to have suffered from the same problems. (A tear in one of the vaccum lines) Where did you buy a new hose from? Did you replace the whole thing or just a small portion? Also, how did you fasten the new hose to the old one?

Thanks much!
 
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#15 ·
Hi,

I used some vacuum line that came with the CVV kit from Tisher and that I never needed to use. And I replaced just a small piece and not the whole thing since it goes behind the manifold and there is no easy access. I think it can not be justified to buy the vacuum hose from the delearship though because it's highly overpriced and it's not any different than the one from a generic auto parts store. I'd take a sample and bring it to Auto zone or Advanced and it's going to be much cheaper. Where my hose disconnected I did not need to fasten anything. There was a small adapter that connected two ends of the hose.

Hope it helps.
 
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#22 ·
LadyMysteriah said:
Alex,

Thank you for this post. My car thru the P 1421 code ans by reading this post with the pics I was able to find the hole, replace the hose with BMW for $27.50, and now I am good to go. I hope that was it.

Thanks,:thumbup:
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No problem, glad it helped :)
 
#23 ·
Sorry to revive an old thread but this is my exact problem. How long after separating the hoses behind the valve cover did you start to get codes? I did it a few months ago and it seems like only recently it started for me. I havent had a chance to check it cause of work, but planning on looking into it this weekend and was just curious as to timeline for this. Thanks for the DIY btw.
 
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#24 ·
No need to say sorry, this thread is here to exchange opinions.

I got the codes like the next day. Before you assume it's the hose behind the VC, check the one by the check valve, check that the air pump kicks in in the morning, check the air pump large hose - those are easy things to do.

These codes might not even be related to the hose behing the VC, but it's a possibility too. It was in my case.
 
#25 ·
Great write-up. I just got these codes on my car yesterday, and like you, received them soon after changing my Vanos. For me, the small vacuum hose that attaches to the valve (#1 in the diagram) must have cracked after I moved these lines around. My hose turned into a hard plastic material that was extremely brittle. The cracking occurred where the hose met the valve connector.

I also checked the line behind the valve cover, but mine was still attached.

Went to a local auto parts store, but no one carried the 3.3 mm hose - everything they had was in standard units. I ended up picking up some 2.8 mm hose (7/64) which was fit (after some minor widening of the hole) over both the valve and the other vacuum hose (#3).

I scanned the OBDII with my Equus 3150 this morning and there were no codes. My CAT and EVA lights are still blinking and I'm receiving a yellow light for the emissions tests. I am assuming I will need to drive the car more before these reset themselves. Anyone have an idea of how long it takes before the system clears these lights (100 miles)?
 
J
#26 ·
Thank you for your awesome and helpful posts!

I have been dealing with the "check engine" light on my car "beamie" for the last three years and P0491 and P042. We have to get cars inspected here and each year before inspection I would take her to the mechanic and ask them to try to get the light off and to get it to pass inspection. Every year they would charge me between $300 - $350 and somehow get it to pass inspection at their own emissions center. Two days after emissions testing the light would come back on. I read your post and we checked our cars emission system. We started by ruling out the least expensive part...the vacuum hose behind the manifold. We could not remove manifold but we did remove air vent cover above the manifold. It gave us more room and we reached and reached until we found a hose that was not connected! We went up to auto zone and bought a vacuum hose for $1, came home and, with mirrors and flashlights we repaired the hose. We disconnected and then reconnected the battery. After three days of local driving the engine light went out and the car passed inspection! $1 that is all it took after reading your post, seeing your photos and reading that newsletter! THANK YOU SOOOOO MUCH FOR ALL OF YOUR HELP!!!!!!
 
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#27 ·
jillsocean, you are very welcome. I'm glad you got rid of the code. :thumbup:

If there are more problems with "beamie" in the future post a thread with a question and we'll see if we can help with a piece of advice.
 
D
#32 ·
I'm just bought myself a 2000 323i. When I bought it the check engine light was on. I took it to a local mechanic being as I dont have a diagonstic tool yet. They told me that there were 15 codes when they first checked. They reset the codes and the only one to come back was a "Malfuction of the secondary air injection system bank 2." Two mounts for the pump are broken, do you think that could be the source of my problem?
 
#33 ·
No, the mounts won't set a code but a break or crack in the plastic and rubber tubing and hoses connected to the air pump will. Check them carefully, especially toward the back of the engine, it's a common problem and an easy repair. The air pump itself may also be the problem, but check the hoses first.
 
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#40 ·
I believe that my pump is going bad. My indications are the screeching sound that begins as soon as I start the cold engine. :)

From the reading that I've been doing today, I'll be replacing the pump and the valve at the same time. Since these are the two most expensive parts, and I don't have the money currently, can I simply disconnect the power to the pump for a month or so while I get the money together? What problems, if any, can I expect?

Thanks,

Etz
 
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#41 ·
Please help!

I have a 2005 545 and I had the vcg changed. Immediately following my ses light came on. Additionally, I get an error message of increased emissions or engine malfunction further driving can damage the catalytic converter. I had the ses checked and I received these same codes P0491/492. Please help! The car idles hard and it's making me insane. By the way I also changed the spark plugs. Any help is more than appreciated.
 
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#42 ·
cbrnshuga said:
I have a 2005 545 and I had the vcg changed. Immediately following my ses light came on. Additionally, I get an error message of increased emissions or engine malfunction further driving can damage the catalytic converter. I had the ses checked and I received these same codes P0491/492. Please help! The car idles hard and it's making me insane. By the way I also changed the spark plugs. Any help is more than appreciated.
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The help is in the first post of this thread ;)
 
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