Hello I just bought a 1999 honda accord and 3 days later the check engine light came on. Checked code with snap on scanner, p0420 catalyst detonation. Took out downtream o2 sensor scuffed it, blew air through it and replaced it. At this time i noticed the cat was missing and there were 2 bubble style resonaters one on each side of the o2 sensor. Cleared code and 2 hours later the cel came on. Replaced both o2 sensors with new bosch parts, downstream was a universal type and upstrem had a factory style harness. Cleared code, 2 hours later cel again. Does anybody have an idea on what to do next, i was going to try the spacer trick which i read on here and pull the battery cable to reset the computer. Thanks for the help looks like a good forum to join.

 

Code P0420 has nothing to do w/ O2 sensors. It means the catalytic converter efficiency is not up to standard. Since you don't have a converter, this is not surprising.

You will either have to install a replacement catalytic converter (recommended), or mask the rear O2 sensor using a threaded spacer between bung on exhaust pipe and O2 sensor. A search for P0420 on this site should find previous threads w/ the link to the spacer solution. I've used this on a Toyota successfully.

good luck