I agree with varsis, if you do it downstream, the data will be skewed because the cat with process some of that waste. If you want a true reading, gotta do upstream.
Understand also that if you install a narrowband air/fuel gauge to almost any fuel-injected engine, the readings on the gauge will be meaningless. The computer adjusts the air/fuel ratio so fast during engine operation that the gauge will do little more than bounce back and forth. For accurate readings, you have to have a wideband when you are talkin about EFI
