Hi guys and gals, this is my first post with your forum and I am asking for a little help with some codes that I have been getting. I pulled a P0300 (random multiple cylinder missfire) and a P0420 (Catalyst efficiency below threshold) from my moms '01 Miata. I figured the P0420 was caused by the misfire. Currently what I have replaced is the plugs(NGK's) and Timing belt and waterpump(I first thought it had jumped time but I replaced it anyways for peace of mind). The misfire is very smooth, not like a plug misfire and it is definately not constant. It almost feels like the coils are 'weak' when it happens, it feels like it loses power (doesnt matter how far you press the throttle, power is cut atleast 50 percent) and when it does happen its usually highway speeds and lasts for only three to five seconds but it is very smooth when it happens, not like a normal misfire. I was thinking about replacing the crank sensor but I am reluctant to do so because it hasnt specifically thrown that particular code. Is their an ignition module on these cars and are they common to go out or am I looking in the wrong direction? Thanks for any input.

 

almost forgot, car has 90K miles, everything is factory down to the fuel filter.

 

my guess is an o2 sensor on the way out (check the air filter first). this in turn screws up the fuel mapping, causing the car to run rich, gumming the cat with unburnt fuel. your 2nd o2 sensor don't get the "right" reading and throws the cat code. hope it helps, good luck.