The oxygen sensors in front and behind the converter are reporting too similar of readings; Leaking fuel injector or high fuel pressure; Cylinder misfire; Oil contamination; Possible Solutions. Some suggested steps for troubleshooting and fixing a P0420 error code include: Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
The catalytic converter is prone to failing on the RX-8. The most common symptom of this problem is a jumpy acceleration. The acceleration should be smooth, but if not, then the catalytic converter would be the most likely cause.