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Possible causes for check engine light

dkpbxmandkpbxman Posts: 1
edited September 2016 in Toyota
A while ago my C/E/L came on and as the car was running fine, I left it alone. It ran perfectly for over 6 months but now I need to get it inspected and the error code fails it.
Connecting a scanner to the car tells me that the codes are P0420 and PO420P which translates to a problem with the catalytic converter.
I know sometimes a particular code may be caused by a different problem.
I really don't think it's the cat so I replaced both O2 sensors, did a smoke test on the exhaust (no leaks) and inspected all the fuses (none broken).
I have reset the C/E/L a number of times after letting it go through it's cycle but the light still pops.

What else could it be?

Thank you,
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