Thanks to all....reason why I asked is that mine is now at 240k, and it barely passed the NOX when I took it in for a smog check at 230k, but the other two readings of HC/CO were very good, just the NOX that was just right at the border....so I have been thinking about replacing the catalytic conv. before the next smog check is due in like 2 years....just doing some research now. I live in CA btw...

I guess I should not have done this, but I poured a bottle of fuel injector cleaner into the gas tank like right before the smog check. Wonder if this could have caused a temporary high NOX reading....

Thanks again...