It's probably catalyst efficiency degradation. The catalyst doesn't necessarily need to be replaced now. Look at the following discussion on the effects of ignition timing on peak combustion temperature (a big factor in NOx generation) and EGT (a big factor in catalyst performance).

http://www.peachparts.com/shopforum/tech-help/117048-successful-ca-asm-emission-test-ke-fuel-system-post833484.html#post833484

Though the initial timing cannot be changed, the rate of ignition advance with engine revs can be changed (slowed) and the vacuum advance can be disabled.

It could also be a lazy O2 sensor. If it were me I would check the duty cycle and scope the O2 sensor waveform as I did prior to my last emissions test. This is also discussed in the above link.

Duke

It is an aftermarket one though. Is it possible that it is degrading so fast? I wonder what kind of paperwork, proofing is required to get a replacement under warranty... I installed it myself.
Anyway, thanks for the information! I will go through it and try the test again without replacemnt of the cat.
Thanks, Bruno

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1992 300TE 160 kmiles

High NoX is typically a lean mixture.(and a result of high combustion temps).....High Hydocarbons are typically the cat.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Half correct.
High HC's are normally caused by ignition misfires, vacuumleaks, & oilconsumption.
Trust me, I do emissions testing for a living.

__________________
2007 C 230 Sport.

I do emission repairs for a living, and feel that the aftermarket cats are no good for the volume testing that they do now. Seen many go south in less then a year.

__________________
Chris Muc

'08 R320CDI 265,000km
'15 GL350 206,000km

"You can fight without ever winning
But never ever win without a fight"

Neil Peart

yeah...I overlooked the obvious there....

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I just replaced my cat on my 560SL with an aftermarket $440.00 vs $4000.00 for a genuine MB. There is a HUGE difference in Weight and Quality between the two. In fact Im so discussted with the DEC cat and Miller crossover pipe I got that I am saving the old MB one to do a full repair on the heat shielding and that they will be put back in when the new ones wear out, which Im sure will be soon. The MB unit is non-magnetic stainless the replacements attract a magnet meaning steel or low grade stainless. The cat size complared to the original MB is tiny. I also belive that I will lose a significant amount of performance with this setup and that the system will be significantly louder. Basically If I had known this was the S@#$% I was doing to get I would have gone to a custome exhaust place and had something realistic made up. Aslo all of the pictures I see of the DEC cat for a MB 560SL are pictures of a genuine MB unit not the DEC cat itself. From all I have read DEC was the most highly recomended of the replacement Cats, due to its mandrel bent tubes, which the Miller crossover pipe does not have, But from what I can see thats not saying much.

John Roncallo