Originally Posted by PDXEvo

The difference between the back of the down pipe and where the CAT is really isnt huge. For the best accuracy possible you should put it in your down pipe but at the back end of the pipe so you dont put it in an area where the EGT's are going to be too hot. The O2 sensor runs really hot as it is and adding additional thermal wear would be bad and will cause false readings.

Putting it into your cat will most likely give you minimal differences in readings and should allow you to tune. I would probably add .1 to all my readings for high RPM tuning just to be safe. Also, make sure you complete a free air calibration on the O2 sensor before plugging it in. You can do this through the Tuner box via a PC connected through Hyperterminal. This is an easier method then doing it manually with the dip switches then needing to update the Tuner box. You should get a reading between 250 and 299. To ensure your accurate after the CAL just go for a quick drive and during a part where you are only part throttle and cruising along at 45 just check your AFR's. You should be right around 14.7. Even sitting at a red light idleing you should be around 14.7. This assumes of course that you havent changed anything for your map in the 0% load area that would cause you to run rich or lean.