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sold summer 2012: 2002 S500 W220.175
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Fed Warranty in Canada is 130,000 km (not miles) and 8 years.
Fed Warranty in US is 80,000 miles and 8 years - I think - to be confirmed.

NOTE: if your cats are making a rattling noise, and you are within the above limits DON'T WAIT - take your car to MB dealer, to have the full pipes and cat replaced under warranty!!!! I missed this myself by 3-4 months! ouch!!

Here is my story... hope you find it useful.

My cats started making noise and given that my Benz is just over 8 years, by 3 months!, and only 120,000 kms, the warranty was not an option for me. I took my car to my local trustworthy mechanic (contact details below) who inspected the 4 cats (2 on each side) and noticed that the 2 coming down (front end) are okay, and only the mid-ones (horizontal) needed changing since they rattled from mild banging with his fist (careful no to damage your cats if still under warranty, since any physical external damage will void the warranty).

Local Benz dealer quoted over CDN$3000 for both sides, complete replacement. Online, I found that I could get the full pipes and 4 cats for $1000 each side, for a DIY. My mechanic suggested universal cats (magnaflow part 53004 M, 2", Metallic Catalytic), for $250 each side, Parts & Labour included, plus taxes. The 53004 M has engine limit of 4.2L, which is enough considering that the S500 has 2.5L on each side (half of the V8, 5L engine).
I believe that the original pipes in the W220 is actually 2.25", just that my mechanic used sleeves that welded to the 2" cat and then used D-clamps to the 2.25" pipe.
More info of magnaflow universal cats at:
Universal 53000M Series MagnaFlow Catalytic Converters

If you're going to weld yourself, and DIY, the cats can be purchased for about $60-100 each. Check out amazon.com, or other online retailers. HottExhaust.com - Magnaflow Car Sound Catalytic Converters

Beware of "direct fit" cats, that are VERY expensive, compared to the weld-on type, which I discuss here. Magnaflow sells direct-fit models 49519 (driver side) and 49420 (passenger side) for almost $2000 EACH on their website - which then sell for about $750 each at other sites:
MagnaFlow 49 State Converter 49519 - Direct Fit Catalytic Converter - Truck Parts
These direct-fit systems may be a good option if you need to replace one whole side, including all pipes and both cats on the SAME side.

Many weld-on cat models and shapes are available. You need to consider the space between the cats, the effect of heat dispersion of being too close, and then the road clearance. In the picts below, one can see that there is little space left between the 53004 M model installed on my car.

The 91004/5 and 99004/5 metallic models are flatter, but their width may make them difficult to fit 2 of them side by side in the W220 - unless you try to place them at an angle (?). All these models have heat shields, even though the original mercedes cat doesn't seem to have a heat shield. An interesting model worth considering would be the 59904 or 59905 model, with an offset-offset design, but no apparent heat shield.
http://www.magnaflow.com/02catalytic...niversal=59905

Whether you chose the 2" - 53004 model, or 2.25" - 53005 model depends on your welding and the adapters you use or not.

My mechanic cut out the old cats, and attached two new cats with sleeves and D-clamps (NOTE: no welding on the original pipes), and everything works perfectly. Only negative is the "look" of the new cats with welded sleeves and D-clamps, which is not an issue for me. In addition, the new cats are not flat on the underside, and you lose about 1" clearance in the center - which should not be an issue if you drive your car on civilized roads. In fact, the back wheel axel/arms and the back end pipes to the mufflers are also lower than the rest of the underside.

Using this sort of sleeves is a great idea, instead of directly welding the new cats to the original pipes, since any future cat replacement would require re-cutting any direct weld on the pipe side, which would further shorten the original pipes.

Below are the pictures of before and after.

Additional notes.
1) When removing the passenger side old cat, the rear bracket/joint was fairly rusted, and even though the mechanic used heat to losen it, it cracked slightly, so he ended up preparing a new "sleeve" joint with about 2" overlap over the original pipes, using a D clamp. On the forward connections which are direct welds, with no brackets with the pipes, the mechanic used the same sleeve connections and D-clamps. This allows the cats to be removed in the future (if needed) without having to cut the pipes once again.

2) the front cats, going up into the engine area, were not changed. mechanic indicated that these don't tend to go bad

If you're in the GTA area, my local mechanic, Alan, is at L & G Auto Exhaust Experts, tel: 905-471-7482, 19 Laidlaw Blv, Unit 11, Markham, Ont, Canada.
 

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Good write up!
Here are additional things to consider when choosing a universal converter:

Choose a converter with the engine size rating that closely matches your car. 53000 and 91000 series converters are rated with a maximum 4.2L and 94000 and 99000 series are for vehicles 4.0L - 6.0L. Converters are heat operated and must reach it's proper light off temperature. Using one too big or too small will cause issues.

Match the gross vehicle weight of the car. Undersized converters will fail prematurely if the weight is exceeded.

Match the o2 sensor location. The location of the o2 sensor should not deviate more then a 1/4" from the factory location. This will cause check engine lights.

Match the pipe size. This will insure proper installation and less chance for leaks.

If your converters have gone bad, chances are the o2 sensors have gone bad too. What ever has caused your converter to plug up, melt etc...most likely contaminated your sensors as well. Make sure you replace the sensors at the same time to insure proper readings.

Additional info:
Selecting The Proper Universal (Converter Basics) - MagnaFlow Catalytic Converters by Car Sound

Converter Failures (Converter Basics) - MagnaFlow Catalytic Converters by Car Sound
 
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