Before and after cat.....

[chrleb2]

Just a quick general question... what should the difference be before and after the cat?

[dfe1]

The difference in what? Assuming you're asking about O2 sensor readings, it depends on operating conditions. At idle and part throttle steady state, the front sensor readings should toggle from about 100 millivolts to 1000 millivolts. The rear O2 sensor readings should be relatively steady at 600 to 800 millivolts. Under wide open throttle, the readings will be steady and fairly close between front and rear. Translated to AFR, at WOT, there's only about a tenth of a ratio difference between the two. This is based on testing with two LM1s installed, one in front and one behind the converter.

[chrleb2]

Yeah, sorry. I meant to add A/F in my question. I had a customer's vehicle that had a 3.0 difference between the two. I figured .2 was the biggest difference you want to see. This thing had some problems.