It might not work. Ive had a CEL ever since i put my header on. I bought a CEL elim from RRM and installed it in the right place and nothing. So i contacted them and they suggested that i put another one on. So i did that and nothing. So i took it to a shop and asked for suggestions. They told me to try the Mustang CEL elim which is a combo of resistors that go inline of the O2 wiring. Still have a CEL.
In the great state of PA you cannot have a CEL code active to pass emmisions. So i had to pay for my exempt stickers. $200 later i have my emmisions sticker and still have a CEL.

Recap
1st CEL elim $20
2nd CEL elim $20
Mustang CEL elim $40
Exempt emmisions sticker $200


Im gonna replace the O2 sensor and install one CEL elim and if i still get a code im gonna get under the car with a multimeter and figure out what resistors are needed to remove the 2nd O2 sensor entirely.

 

im goin to bump this cause i have the same problem even after gettin the recall done i to live in pa and reset it right before gettin emissons done and passed with a turbo you should of did the same i have a test pipe tho sittin in my room if i put this on will it fix it or should i go get mitsubishi to give me a new cat to make it easier for me to get those cat bolts offwhen puttin the test pipe on since they would of broke them free while puttin the new cat on

 

ok. First try punctuation. Its fun and helpful.

Second i did reset my ecu before i went. As soon as the learning period was over my CEL came right back on.

A test pipe will not fix the problem. The second o2 sensor is reading the exhaust from the first cat (which with a header we no longer have). A test pipe just removes the second cat.

 

Well I have nearly 120K miles on my car and this little code just popped up for me. I resetted it last week but it came back last night.

So it's the cat and not the O2 sensor...the reason I ask, I know my thermostat is stuck open and my car is running cold (ie rich) which probably has extra fuel going out my exhaust and probably jacking up my cats and o2 sensor.

I was thinking of just replacing my thermo and 1st o2 sensor this weekend and evenutally get myself an aftermarket cat and catback.

nice info info guys.

 

back to life again
so, i have read like every post on this 0421 code.
If the car is idles up to 1500 and then creeps back down to 700 and bounces around between 550-700 on a cold start with the a/f reading about 10/1, but after driving it around gently for about 5 min, everything is fine, idle is good and everything works and is where it should be... what's the deal?
I am thinking the 02 sensor heater core/wire is bad and just need to replace the front 02 sensor... any help would be appreciated

 

mine just came on. same problem as you guys have. brought it to a shop. says here

checked & found dtc# p0421 stored in memory-warm-uo catalyst efficiency below threshold bank one.
scpe checked engine sensors & found rear 02 sensor voltage switching above & below threshold of 45mv too often- indicating material inside cat converter unable to reduce emissions properly. recommended to replace front cat converter w/exhaust manifold.

so hopefuly wen i go down to vercacom mitsubishi next week when i have time, and hopefuly covered under warranty. i have 70k on the car at the moment

 

oops.. below threshold of 450mv