I just went through the pollution gauntlet here in Colorado....Turns out my cat was bad. Unfortunately, My engine is a '97 and has the OBDII system...so this whole story probably isn't going to be all that helpful for you....but...

First time I went in, I failed. Which wasn't entirely surprising to me because my ECU had been throwing O2 sensor codes at me. While driving up at really high elevation (around 11,000 feet), I would get a code that basically said my O2 sensor readings were "ambiguous". Not sure exactly how to interpret that, I would just clear the codes and go on with life. The check engine light only came on at those high elevation...so I really didn't think too much of it.

Also, after failing emissions that first time, I found out that the EPA has LOWERED the acceptable emissions level...for all cars. And not by a small amount either; the acceptable levels for HC and NOx, for a '97 SUV, have been lowered to 1/3 of the levels that were acceptable in 2008. So...that was a BIG surprise.

After playing around on here and talking to my local Yota parts department guy, I cleaned my MAF and replaced my primary O2 sensor (the one before the cat). This did have a noticeable effect, during the second emissions test. My readings dropped by around 15%....but, I was still getting twice the acceptable levels for HC and NOx. Which was really disconcerting, because these were the lowest levels that I've ever gotten from this motor (I kept the results from the previous tests).

Now desperate, and thinking I would never get this motor to pass emissions, I bought an OBD2Link and software package for my laptop. This is now a must have tool for '96 and newer vehicles...this thing is great. I can connect my laptop up to my ECU and get actual real time readings from the engine sensors. Not just read codes, but actually see what the sensors readings are and how they are responding, what the timing advance is doing...how the fuel trims are responding.....all while the engine is running....it's really cool.

Anyway...the very first thing I noticed, after connecting the OBDLink up (and even before fully understanding the power of the software package) was that my O2 sensor readings were flat. My truck has an O2 sensor before the cat (the primary) and a second sensor after the cat (the secondary). If the catalytic converter is working at all, there should be a change in the reading from the primary to the secondary....I had nothing, both read 0.8 volts at idle, and did changed but stayed equal to each other when I gave it gas. So, I was pretty certain my cat wasn't working at this point.

So...a little online research later and I found out that flowmaster cat's are basically junk. Which sucks 'cause I had put a flowmaster cat in there when I did the 3.4l motor swap a few years back. I couldn't find one positive customer review of the things online.

My next move was to install a Walker catalytic converter (which got good reviews and costs about the same as a flowmaster). I hooked up the OBDLink and now I'm getting a 0.2 volt difference between the two O2 sensors at idle (primary was 0.8v, secondary was 0.6 volts. Then it was off to the smog police....for a 3rd try.

The difference was HUGE. My HC, CO and NOx exhaust gas readings are now a fraction of what is allowable. Needless to say, I'm now legal (and re-registered in Colorado).

A few things I learned from my research/experience....
- with catalytic converters, bigger is definitely better. The bigger the cat, the more free flowing it is, AND the more effective it is, and the longer it will last. The cat I put into my '93 4Runner with a 1997 3.4L v6, was sized for a 5.7L motor.
- I also learned that catalytic converters are specifically made for 1996 and later vehicles with the OBDII system. I'm not sure what the differences actually are, but most aftermarket manufacturers sell standard cats and OBDII cats.
-The final lesson was that the flowmaster cat I had originally installed was probably never doing anything to effect the exhaust gases. Before replacing the cat, I was getting almost exactly the same readings as I had got right after doing the motor swap (which doesn't pass the current standards).


Sorry for the long post...but, I hope that helps.