2002 Protege, code P0421????

[mr.bigg]

Hello all!! I'm new at this but here goes....
I have a 2002 Mazda Protege LX, 2.0 liter. I was driving with the cruise engaged when the car felt like it lost power and the CEL started flashing. Took the cruise off, depressed clutch, revved engine, finally power was restored. CEL stayed on constant. Finally took it in to the shop and they advised that I had a multiple cylinder misfire code. They wanted 700 for a basic tune up well. I decided to replace some items on my own. I have replaced the PCV valve, air cleaner, plugs, wires, and the top O2 sensor. I ran injector cleaner through (Lucas) two tanks of gas. Then I replaced the coils (both). After each item, I reset the CEL. The light is back on. I am now getting a P0421 code (reset and verified twice). The code indicates a warm up catalyst system (RH) efficiency below threshold.
Probable cause:
aa1, exhaust leak (cant hear one),
aa2, check other service codes first (none),
aa3, faulty catalytic converter.

Idle bounces occasionally (below 500 to 1000, levels at 750). Some hesitation.

I am considering replacing the lower O2 sensor (at 108000 it is probably time anyway).

Not sure what to try next before I take her back to the mechanic. Ideas?????

Thanks

 

[justanothermp5]

well just a pointer, normally when you have a multiple missfire, the car just dumps raw fuel into the cat. converter
this, in some cases, can ruin the cat converter, so even if you fix everything else the cat. would still be bad and thats what cold be causing the CEL

 

[mr.bigg]

how to check the cat?

Thanks for the reply!!
Is there any way to check the cat converter without just replacing it?

 

[Canadian-ES-GT]

Egr

I had very rough idle for a couple months when I finally brought it to my mechanic. I had a P0300 code which is mutiple missfire. He unblocked my EGR and now it runs fine. I don't think that it's the same problem but my mechanic also told me about the EGR getting stuck closed (mine was stuck open) and it includes a loss of power. I would maybe consider your EGR.

 

[justanothermp5]

^thats a very common issue so check that one too

and im not sure if there is a way to check for a bad cat. converter
normally they smell like rotten eggs or sulfur when they go bad but thats not always the case

 

[tzulinke]

hey I just replaced my stock headers wit aftermarket headers getting the code P0421.I think that it is just because I do not have a pre-cat anymore. let me know if i am right or wrong


Thanks
T-rev

 

[P-Funk!]

Yup. You need a non-fouler solution. Search. Much discussion on it.

 

[Mrhiguy]

Non fouler trick works. I have about 5000 miles on it with no MIL