Code P0420 again.

1999 Chevy Metro, 3 cyl. 144,500 miles.Scan picked up a P0420. Further inspection revealed downstream oxygen sensor was dead. Replaced sensor and PCV. Fine for a week then scanned P0420 again. Was told by professional mechanic that Cat has some build up that might go away after a couple long highway trips. Is that bad advice and should I check something else before replacing Cat?
Thank you for any advice.

Do you have a scan tool of your own to compare the ho2s11 and ho2s12 voltages?

How did you determine the catalyst monitor was 'dead'?

I'm of the mind that your cat is shot, if you can observe some scan tool data for me we can determine whether or not this is the case. I don't see how under the circumstances your professional mechanic reasoned that there is just some 'build up' on the cat that will go away.

If you get a scan tool hooked up drive the car around the block a few times and get it to operating temperature, then view data:

Your air/fuel sensor (o2s11) voltage should fluctuate regularly between 0.2-0.8v

Your cat monitor (o2s12) should stay pretty static and fluctuate very little and not as often as the fuel ratio sensor (o2s11). If the voltages for HO2S12 are all over the place like HO2S11, the catalytic converter is bad and must be replaced.

-MechanicMatt

Thank you for your good advice. I'm very sorry it took me so long to reply, I suffer from severe procrastinitis. Made up that word so I wouldn't have to call myself LAZY.

I agree with Caprice. The cat probably melted down.

Thanks, Caprice and Johnny Mullet. You were both right, the cat has been replaced and code is gone.