My 1998 Accord EX continually gets a PO420 OBD Code (Catalyst efficiency Below Threshhold). I have approximately 210K miles on it. I have already changed out the EGR valve and associated drilling work, about 60K miles ago. However, occasionally I will still get the PO420 code. It comes and goes. Any ideas on the cause? :runaround:

With 210K miles, your three-way catalyst (TWC) may have lost enough efficiency to make it require replacement. My manual states that if that is the only code that is set, then a scan tool is required to determine if the TWC is not functioning properly. It is a pass/fail type test, and the only part replacement at the end of the testing is the TWC.

Before you replace that expensive of a part ($380 wholesale), I'd get it diagnosed properly so you aren't just swapping parts.....hope this helps...Glen

Thanks for the response. I'll get it checked out. With that many miles, I thought that I may be getting an eventual failure. At any rate, I'll have it checked out before I spend the big dough. Thanks for your help.