Sorry to bump this old thread, but thought I would instead of starting a new one...
My reasoning for posting/bumping instead of following/solving is that I read in a different thread that the EGR can cause this code?
Here is the back-ground. Bought my early `02 P5 with 212,XXX miles on it about 3 weeks ago. CEL was on, previous owner had a reader and told me it was a bad knock sensor. No other codes to his knowledge, however, he had just purchased the car 12 days prior from a mazda dealership, so he really didn't know the history of the car...
So, while waiting for a few things with the title work to be completed, I decided to clean the EGR since it seemed to be an easy task and the car had a slightly rough idle. In doing so, I accidentally took apart the sealed part. IDK what I was thinking, I was kinda out of it. So, did I mess up the EGR?
Fast-Forward, knock sensor came in, turns out it wasn't even there! As in, I couldn't find the old one to remove because it wasn't there. Found a spot on the wire-harness where I thought it would plug into and it did. Found a spot on the back of the intake manifold to screw it into, done...
CEL was gone for almost 30 miles, then back on. Thought I probably had the knock sensor in the wrong place, so I just decided to forget about it for the time-being...
Replaced the plugs, installed the OEM intake, reset the ecu, CEL back on after 25'ish miles. Oh, and there was oil on the plugs/plug boots, bad valve cover gasket?
Finally stopped by idiot-zone today and had them run the code. P0421. More information, if the car sits about 8 hours, she spits out some nice blue'ish white'ish smoke. Ok, a little more than "some" a lot! Oil? Or fuel?
So what should I do in what order? I had planned to changed the plug boots, just haven't done it yet. Not sure if she's miss-firing on the interstate or if she's just really slow compared to my BMW. Would it be obvious if the car was miss-firing?
Here is what I'm thinking...
Plug boots (was going to do this anyway)
EGR Valve (if it doesn't cure it, pull it back off and return)
Fouler mod (seems cheap and easy)
Replace cat (eek!)
I'm confused that people have stated "coil packs" separate from "plug boots" I thought they were one in the same on these cars, am I wrong?
ALL HELP APPRECIATED!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
EDIT: Now that I've read almost every thread with "P0421" in the title, I think that miss-fire'ings could be the culprit. One poster mentioned some "bucking" by the engine, I feel this at times, just thought thats how she ran. Will most definitely replace the plug boots, still confused on "coil-packs." Please help! Since my cat is most likely plugged, I'll go ahead and start on the anti-fouler mod as well. Does running a plugged cat do any damage to the car? Or just the environment? I'll leave the EGR as the last option...
EDIT II: Ok, so 2 have "coils" and two are "slaves." Correct?