Eliminating P0420 code on 9-5 with performance down pipe

[MI-Roger]

I have been plagued with a recurring P0420 CEL code (low catalyst efficiency) since late November and decided to try the O2 adapter from Big Daddies Garage to eliminate this CEL associated with using a race cat after ~45,000 miles. I FINALLY got it installed this morning!

I initially ordered the 90-degree model only to discover it cannot be threaded into the bung on my 9-5 with a GS down pipe without removing & re-installing the down pipe. This is due to interference with the left side jackshaft while threading it into the bung. I exchanged it for a straight style, which took much longer than I expected, and by the time the new adapter arrived Winter had hit in full force delaying installation. I am a wimp, I don't like laying in snow in 10 degree weather working under a car dripping with road salt residue.

The straight style is no piece of cake to install either. I finally got it installed by threading the O2 adapter halfway into the bung, then threading the O2 sensor into the adapter, then tightening both while under the car. My first attempts were to thread the sensor into the adapter on the bench, then thread the longer assembly into the rear O2 sensor bung. The rear engine mount is just enough in the way to make this impossible. Dropping the down pipe and using the more expensive angled adapter might be easier.

My P0420 problem has gotten to the point where I am receiving CELs once per week or more, which is one or two per 500 miles driven. I should know soon whether this $15 adapter is the trick I need. If not, Phase II is to have a new trailing piece made for my down pipe (the mostly straight piece of pipe that comes with the GS down pipe) by a local custom exhaust shop with a monster truck sized Catalytic Converter welded into it and a O2 sensor extension cable purchased from the 9-3SS page of GenuineSaab's web site to reach the rear O2 sensor which will then be nearly 2 ft further aft.

I am keeping my fingers crossed. Owners of other vehicles say this trick has worked for over two years on their vehicles with custom exhausts. At 11 years of age and nearly 300,000 miles I can't see the sense in buying a new down pipe or new factory catalytic converter for my 9-5.