I have an '05 STi w/ cat-less exhaust and used the mechanical fix. I ran for about a month before getting my Cobb AP and never threw a CEL. The mechanical fix is using a pair of spark plug antifoulers threaded into one another with your O2 sensor threaded into this. One of the antifoulers you drill out w/ a 1/2" drill bit so the sensor fits through. This allows the sensor to get a real reading from the exhaust, just a reduced one due to the 1/4" hole that the exhaust gasses can pass through to the sensor. I used the "Help" brand antifoulers. I bought both the 42009 and 42002 sizes. I drilled out the 1/2" hole in one of the 42002 and threaded this into one of the 42009. The 42009 then threads into the coupling on the exhaust and your O2 sensor threads into the 42002. Make sure they are tight w/out stripping the threads. Maybe use some loctite to prevent vibrations from letting loose. Mine has been in now for nearly 4 months and is still working perfect. Good luck and if you have any other questions about this, let me know.