Looking for a little advice or ideas.

I have a bone stock 2001 ES 2.0 and about two weeks ago I got a flashing CEL. Ran the code and got back a P0300 for a random misfire and P0421 for low cat threshold.

After trying the things below I still get a flashing CEL about once every other day, so if you have any suggestions please let me know.

Things done thus far:

Cleaned EGR Valve
Replaced both front and rear (up and down) O2 sensors
Changed plugs
Bench test coils and wires, which came back fine
Inspected the Cat, looks to be fine

 

It's wierd seeing this on here because I am having the same problem also. I changed my plugs and my coils and the Po300 code went away, but soon after the PO421 code showed up. From what I've read the only thing that I would reccommend is changing the cat. If both the o2 Sensors are changed the only thing that would set off the downflow sensor would be a faulty cat. I'm looking at changing my cat here in the next week or so, so I'll let you know how that turns out.