Two mechanical CEL fixes are described below to address the P0546 and P0420 CEL codes. There is also an alternative electronic CEL fix which is much MUCH neater and obsoletes this mechanical fix.
P0546 Cel Fix
The CEL fix procedure for fault code P0546 is described within the Uppipe FAQ. Note this does not fix the more common fault code P0420 which results from a decat downpipe.
See below for a few snaps from my install:
P0546 CEL Fix - Euro WRX MY03
P0420 CEL Fix
For P0420 you should either purchase a plugin 'CEL Fix' or attempt to search the 'net for an alternative resistor-based solution. One alternative from 'ParticleMan' is described below.
This plugin 'CEL Fix' is actually quite easy to do. Obtain 2 spark plug anti-foulers which are Part # 42009 from autozone. They are of "Help!" brand. Drill one of the anti-foulers with a 1/2 inch drill bit so the hole is capable of fitting the 02 sensor through it. Now screw on the O2 Sensor to this piece that you have just drilled. Make sure you put the included gasket in there. Now screw this assembly to the other anti-fouler you have that came in the packaging. Now plug this new longer assembly into the midpipe/downpipe.