Let the O2 Sensor connected to the ECU but outside of the exhaust.
Some guys also use a lambda sleeve
Use google. It's only important that the signal of the sensor is straight (a cat is only a o2 memory, so the O2 after cat diagnostic only looks if it's different from the before cat O2. Newer ECM sometimes detect also the offset/delay between both sensors during enrichment but this seems not be present very often).
