mechhound

I have a 2000 CRV that has 287,000 miles on and the head was very recently replaced on with a rebuilt head from NAPA. The engine suddenly started running terrible and the independent shop diagnosed it as a bad valve. The CRV was only driven about 5 miles with the terrible running engine. The engine used very little oil with that many miles, a half quart or less between 3000 mile oil changes.

Anyway the check engine light comes on after about 15 miles of driving. I cleared the code by unhooking the battery and after about another 15 miles the check engine light comes back on. The engine runs great otherwise.

I just bought a code reader and it shows a PO 420 catalyst system efficiency below threshold. What would be the likely fix for this? Will it hurt anything if I just drive it like this? FWIW we have no smog testing here. Thanks for any insight you can give me on this.