Failed emmission test on my 1998 BMW 740i


I have failed the emission test 6 months after replacing the catalytic converters. I was told to replace bank #2 which I did again with a Magna -flow certified and approved cat for 740i BMW’s. All other o2 sensors, fuel, smoke exhaust tests etc… were run twice by a Gold Shield repair location and I was told my engine was running at 15.2 efficiency which is excellant. What should I do to pass the emission test to get my registration?

What needs to be known is why the car is failing.
Failing due to high HCs, NOX, CO, ?

High nox… 15 mph = 264
25 mph = 892…failed

I was advised I had a lose intake hose causing a leak and that it had burned bank #2 and to replace it again… which I did and then had it retested. I failed to pass the emission test again.

It reported high nox… 1200 but engine co2 is at 15.2%

High NOX often points to a problem with the EGR system.

I don’t know if your BMW has an egr system, or if it uses cam-phasing to control NOx levels, but you should have this looked at.

Also, after looking at the Magnaflow website, it looks like your converters are only warranted for 6 months, so you might want to buy a better product next time if the converter turns out to be the cause of your issues.

California converters seem to be covered for 4 years, so if you are in California, and you still have the receipt, you might be able to get a replacement.

Also, it could be carbon build up in the intake tract, or a leak somewhere in the intake manifold. You could try a hot soak of seafoam, or a double dose of techron fuel system treatment in the gas tank, and then drive the whole tank out in one shot, and that should eliminate all the important fuel system deposits.


My BMW does not have an EGR System…and the Gold Shiled Approved Repair Station confirmed that the car was smoke tested and there were no leaks anywhere in the intake manifold. it was tested twice but it was not tested for carbon buildup. How would you clear that out? Is there a confirmed sign that could be the problem before going to that extent? What else can yo think of?

In case you didn’t see this post in your other thread in the General Discussion forum:

Here’s a link where someone in Texas had the EXACT same issue as you do, and read what was recommended that the owner do:


So try it.
The advice is free.
The cost of a bottle of seafoam, or a couple of bottles of techron is about $10.

For $10, I would just pour in the techron fuel system cleaner, and give that a shot.
Heck, if you have a low tank, maybe about 1/2 a tank, and throw in 1 bottle of either techron or seafoam, and just drive through the remaining 10 gallons of gas, you would get the same effect as driving on a full tank.

Since you car gets somewhere between 17 and 24 mpg, 10 gallons of gas means a 200 mile highway drive, and it should be well cleaned out at that point.


I would look at some type of malfunction in the secondary air system that is preventing the cats. from doing their job as well as they should be doing it. There are known issues with clogged secondary ports in these engines that just may be causing high NOX as a side issue (meaning preventing oxygen from reaching the cats.)Just an idea.

I do have a problem with this idea as it is normal when these secondary air passages are plugged they set a code (per BMW Service Information).

Check the engines ignition timing, if such a thing is possible on these cars…An overly advanced timing will cause the NOx to soar…

How many miles on this car?

Hey Oldschool,

Where are these ports located in the BMW?
I’m just curious.

In the cylinder head?


The ports are within the heads, the TSB is SI 11 05 04,engines affected are the M62TU and S62. This TSB (SI in BMW speak) details a procedure on how to clean them out. What you will be removing is carbon buildup. The TSB does not say high NOX will be a result but I could see if the secondary injection system was not working as good as possible the cats. themselves may not work as good as possible,resulting in high NOX.