The 1ZZ on the Matrix likes Denso Iridium plugs, not platinum. If you have the 2ZZ, you definitely don't want to use platinum.I scanned my Matrix a few days ago and got code P0420. It said that the forward catalytic converter is bad. I priced one at Advance, Autozone and O'Reilly's and they are in the neighborhood of $375 for a direct fit replacement. I looked on eBay and they are somewhat cheaper and supposedly also direct fit. The car runs fine as it is with 205k on it. I reset the code, changed the plugs out for Bosch Iridiums because it had Denso platinums in it and the code hasn't come back, yet. Any suggestions would be greatly appreciated!
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Every P0420 I've seen is from a bad downstream oxygen sensor, not a catalytic converter. If you haven't seen the code come back though, ignore it until it does.