I have tested before and after the cat and have found EXACTLY the following... No difference in reading, but there IS a difference in response time...
Originally Posted by wrxsti.l
Most tuners (and dynos) put a removeable WB02 sensor in the tail-pipe during tuning/logging/dyno. I have compared my LC-1 directly behind my turbo to a WB02 in the tail-pipe and there was almost no difference in actual AFR - however, there was a definate difference in response.
At a constant throttle, AFR is almost exactly the same, a quick stab of the throttle will show the difference in response - and the WB02 in the tail pipe felt about half a second behind.
Saying that, many reputable tuners have and still do tune with the WB02 in the tail-pipe, so it can't be as bad as it is perceived to be - can it?