Truth is it has always been illegal to replace your Chevy Impala's CAT (or any vehicle's CAT for that matter) with an non matching Catalytic Converter (2002 model year CAT instead of 2003). However in the past smog check technicians in California were not being monitored as closely as they are today and so they had much more wiggle room. If the CAT looked like it fit, it was of the same make and model as the original, they passed it.
Today the BAR is making sure every smog check technician pays very close attention to a vehicle's CAT and ensures it is the correct CAT for the vehicle; the smog technicians license depends on it, literally. With the new STAR smog check program smog technicians have to make 100% sure they are following the smog check test procedure to a "T".
Unfortunately at this point it appears your only recourse is to purchase a new dealer CAT. You may choose to contact a State Smog Check Referee for an exemption, however our thoughts are that they will more than likely not allow that since the CAT is an "available" dealer part.
