Gents:

Just my experience with this critter. Had to change out the Dist assy followed by timing check and reset. Got the 0420 code once, reset it, not recurred until cold weather, then it came on every day after engine cool down and restart. Read somewhere that timing could set it off. Checked it, it was off, then reset it, but with a 2 degree advance. All is well now. For what its worth....