Hi there,
This is an excerpt from a post I had made previously following some issues getting through emissions:
My problem was NOX, rather than HC, CO2. The problem your friend is having is the opposite of mine, but the solutions are the same (just using the reverse...make sense?).
The air bypass on the MAF (or AFM depending on how you like it) might be very telling. Mine was way out of spec on the negative side. Your friend could be way out on the positive end. This is tested via the O2 Sensor...see below.
Adjusting the FQS on the DME isn't the best idea in the world, but if you just want to get through emissions it can be tweaked, then tweaked back.
There's also some fuel additive you can use that, allegedy, is Garaunteed to Pass
http://www.autobarn.net/ch05063.html
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The 3 things that made the world of difference for me likely would for you.
Adjust the DME (adjust it back after the test)
Adjust the air bypass on the MAF
Replace the cat
Unfortunately all 3 things were done at once, so I don't know which one makes the biggest difference, but I'll give you my thoughts on them all.
Adjust the DME
Actually, it's the FQS (Fuel Quality Switch) in the DME that you're adjusting.
http://frwilk.com/944dme/fqswitch.htm - this tells you about the FQS adjustments. I put mine in position 4, you would likely be looking to position 2.
http://www.geocities.com/motorcity/1760/dme.html - this tells you how to access it.
http://www.promaxmotorsport.com/fqs.htm - everything on one page.
Adjusting the Air Bypass on the MAF
With the car running you need to back probe the O2 Sensor to find out the voltage. Anything over 0.5v means you're running rich. Below 0.45v is running lean (mine was at 0.32v when we started).
On the 944 the by-pass is adjusted using a 4mm allen key to a bolt on the MAF.
http://frwilk.com/944dme/afm.htm - the bolt head can be seen on the first picture of this page, lower right corner. It is covered by default. The cover is removed by screwing into the top of the cover with a sheet metal screw (just a couple of threads into it), then removing gently with a set of pliers or vice grips.
http://frwilk.com/944dme/lambda.htm - This gives you the voltage it should be.
Turning the 4mm allen bolt counterclockwise (I recall) will lower the voltage you read at the MAF. Take it slow and let the reading stablize before continuing to adjust. The reading will also jump around a bit so you need to average it.
Catalytic Converter Replacement
I would pick up a universal cat, then take it to a muffler shop and have them install it. You should be able to get one and hae it put in for close to $100. Might be a little more, but shouldn't be much.