My 2006 Toyota Prius with 280k miles is throwing the P0420 code, which means that it (probably) needs a new catalytic converter. I live in California, which makes life much more difficult to replace a cat. The direct fit CARB-compliant catalytic converter cost about $1500 in the Toyota dealers (from I could search, still didn't asked to one directly), which is way too expensive compared to welding a universal catalytic converter (about $300). Will it still pass smog with a universal catalytic converted welded in?
In short: no.
To pass smog in California (and other CARB adopting states) the catalytic converter must be the one from the dealership or one that is in the Aftermarket Catalytic Converter Database. The catalytic converter must be one of those listed for the Toyota Prius, but unfortunately there is none for the Prius.
For now the only possibility is to try other solutions like using a catalytic converter cleaning additive, washing the catalytic converter or make a tune up in the hope that one of those will help.
For those curious with the end of my ordeal, after months of poke and hope, I changed spark plugs, poured Cataclean and more recently poured Seafoam and it worked! At least long enough to me go to smog and have the car certified. After 3 miles it came back again :-(
Try a junkyard/recycling center before buying the part new. You can find one in good condition for much less. If you want a new part, ordering it online is usually cheaper. This is not something that the dealership needs to do. Have your mechanic remove the old and install the new. Then delete/reset the codes.
Has your mechanic tested the O2 sensors?