P0420 Code Question...

I recently purchased an '00 miata, 76k miles. About a week after purchasing it I was having really rough starts, occasional near-power loss upon start-up, and once in a while the car would shake horribly when the engine turned over, & I got a flashing CE light. Multiple engine misfire code, & after reading up on this problem amongst other miata owners, I replaced the coil pack which eliminated the problem. I'm now getting the P0420 code, although the car has been running well. Damaged cat due to the coil pack failing? I'm looking for a 'next best step' to fix the problem for good.. any thoughts? Thanks :queen:

Yep, you hit on the most likely culprit. You can try checking the O2 sensors, or using an anti-fouler, but its a good possibility that the cat got ruined.

If the coil got weak, the cat is probably gone. I had a similar situation with my car, replaced coils, plugs, wires, cat and rear 02 sensor.

Check this also:
http://www.miata.net/garage/egr.html

thanks everyone! I'll get to working on it ;)

402 != 420

402 is bad EGR which can be a dirty EGR or clogged vacuum hose fitting on the intake manifold.
420 is O2 sensor (bank 1, sensor 2) bad values.

For 420, go get it real hot, clear the code, then monitor the 2nd O2 sensor voltage. At idle, after everything is warm, it should remain < .4 volts while the 1st O2 sensor bounces between .1 and .8 volts.
If you get 0 volts off the 2nd O2 sensor, then the sensor is bad. Replace it (Rock Auto Densa plug-N-play $65). If the 2nd O2 sensor's voltage follows the 1st, then it's a bad cat. I'd wait until the light comes on again before replacing it. My car throws 420's when ever I do multiple short trips and never get it fully warm. Once I get it hot, the voltages look good.