reply to Anonymous_
Re: catalytic converter? P0240
99% of the time, a P0420 means you need to replace the deficient converter. O2 sensor failure *will not* cause the P0420 to set, but rather will give sensor failure codes. Monitoring the catalyst efficiency requires both the upstream and downstream sensors to be properly operating, so it is generally a very accurate failure code.
Nothing wrong with clearing it and seeing if it comes back, that's prudent.