P0421 what do you think of this

[Bach4660]

I found this on another site, what do you think?
This fix will basically get rid of a SES due to error code P0421 (Warm Up Catalyst Below Threshold (Bank 1)) caused by either a test pipe/down pipe (No Cat) or a High Flow Cat. When this light comes on from your install of a new exhaust/header/etc, be aware that this has no effect on the performance of your car. its just a stupid reminder that the o2 sensor is finding a problem involving something to do with the stock catalytic converter. Yes, by the way, an o2 Sim would do the trick too, but why spend upwards of $50 and have to deal with the cost/install/and problems associated with that kind of fix when you can just do this for about $5 bucks?

Pre-Install
- You might have space for only 1 spark plug anti-fouler, sometimes using 1 will work, sometimes not.
- Having an electric drill helps out tremendously, those battery operated ones dont have that much power; they will still work but will be a pain
- This may still not solve your problem of the light; a small amount of people that try this 'quick fix' still cant get the light to go off, this could be due to having multiple error codes. It SHOULD fix P0421.

STEP 1
-purchase 1 package of 2 spark plug anti foulers at your local auto parts store
-18 mm size
-made by HELP! (Part #42002)


STEP 2
-disconnect the negative terminal on your battery. this will reset your ecu
-take one of the anti foulers and drill out its center using a 1/2 bit, this will increase its hole size and allow your stock o2 sensor to fit. If you can only fit one under there, use this one.



STEP 3
-screw the drilled spark plug anti-fouler on top of the original
-be sure to use some kind of anti-seize on the treads. in case you need to remove your fix with ease



STEP 4
-jack the car up or use a lift
-unscrew your stock o2 sensor and place the foulers in
-use anti seize on the threads
-clean your o2 (by wiping it off with a paper towel DO NOT GET IT WET!)




STEP 5
-reinstall your o2 sensor
-use anti seize on the threads



STEP 6
-reconnect your battery (Resets your ECU)

[AU02LX]

Tried this yesterday, seems to be working so far. I've had the P0421 for about 4 months now, been just checking & resetting it every 3-4 weeks. Usually comes back in 10-15 miles of driving. Tried this mod and checked it with the OBDII monitor. Really changed the pattern on the rear oxygen sensor. Have driven about 40 miles now and CEL has not come back on yet. Will post again in a week or so with another report.

[AU02LX]

After a week and about 200 miles, no CEL seems to have the ECU fooled.

[Bach4660]

thanks for the posts, I was really curious if it would work, it did make sense. I never got a chance to try it on mine as I ended up trading the PV for Mazda B3000 truck

[JoshjaniMPV]

I'm extremely interested in this, as I've been plagued by the 0421 code for some months now. I actually started the process, but couldn't get the o2 sensor off. I need a jack with a higher lift and a wrench I can get a pipe on, I think, so's I can apply more muscle (which I guess I don't have much of!!!)

I do have a question, though- I put this mod on the sensor of the pre-cat giving the problem, right? In case of 0421, it's the right-front one, correct?

The pics of the mod I've seen are when guys are installing headers and removing the stock cats- I just want to try it with my stock equipment.

Looking forward to more info on this- I'm ready to go buy a new jack and get it done!

JC

[steve12955]

Hello,

I, too am interested in doing this. My car runs perfectly but the code PO421 persists. All O2 sensors have been replaced. I live in MA and effective this year state inspection stations have adopted a new system that doesn't even use an exhaust sniffer - it's all monitored using the diagnostic port under the dash. Brought my '00 MPV, 175K miles in today for its annual inspection but disconnected the battery and cleared the code last night. However, I got a rejection sticker since the system detected that either a battery disconnect or recent repair work had cleared the code. Can't return for inspection until I've driven 50-60 miles and there's enough data stored in the ECU - but by then the code will be back, I'm sure. I assume that the O2 sensors BEFORE the catalytic converters (that is, UPSTREAM and located up high on the manifolds) are the ones that are re-installed into the anti-foulers. Is this true?

Anyone else tried this and does it work?

Thanks,

Steve

[JoshjaniMPV]

[Bach4660]

the 421 code is the after cat 02 sensor, which is why this mod is fairly harmless. The front 02 sensor monitors Air and fuel and makes changes, so you really don't want to mess with it, but the last 02 sensor, the after cat merely samples the exhaust to make sure that 80 or 95% badness has been removed. So putting the post-cat sensor into the anti-fouler samples less and will give it a clean reading. No harm to the engine, you just kill a few bees etc.

[steve12955]

All,

If the light reappears before I have it reinspected again, I'm going to try the anti-fouler set-up and will let you know what happens. I've driven the car 450+ miles since the failed inspection (one long trip to / from NJ, a few short around town trips) but when I brought it in again today for reinspection, the technician showed me on his screen that one of the cats and the EGR system were still "not ready", meaning that it hadn't gone through enough cycles yet and it couldn't be tested. Thought for sure that by now it would have enough data.

My car is a CA-spec model, even though I'm in MA, so I'm told the PO421 refers to the cat on the side where the #1 cylinder is - closest to the firewall. That said, I'll install the anti-foulers in the bung after the cat (downstream).

Regards,

Steve

[JoshjaniMPV]

I did this mod today, but haven't had enough time driving to see if it's effective. Many Mazda owners around the 'net swear by it, though!

I put the non-fouler spacer in the bung right after the right hand pre-cat. Plenty of room for it after all, and PB blaster was very helpful in getting O2 sensor out. I used some anti-seize lube on all threads, and everything went back together smoothly.

So- we'll see. I'll keep you posted!

JC

[thm73]

Ugh- I am dealing with this also - on my 03 MPV -

Any updates on experiences with the nonfoulers working well?

[steve12955]

Hello,

The non-fouler trick seems to work. My '00 with 175K was also plagued by a recurring PO421 code. I disconnected battery for 15 minutes to erase code, installed the non-foulers for both downstream O2 sensors and drove the car about 80 miles with numerous on / off cycles over the past couple of days to be sure there was enough data stored in the ECU. Had my car reinspected this morning and everything passed! (After failing three times over the past week.)

Due to interference with the transmission, I was only able to install one drilled-out non-fouler on the Bank 2 side (the side closest to the radiator) but easily installed two non-foulers (one drilled, one intact) on the Bank 1 side (the side closest to the firewall). The non-foulers were available at AutoZone (their P/N 42002) for $3.00 for a pack of two. So, for $6.00 and about twenty minutes' work - problem solved. Sure beats new catalytic converters...

Good luck,

Steve

[JoshjaniMPV]

I'd have to agree so far that this mod is good for p0421.

I installed the non-foulers on only the right cat (near firewall). I did it on Saturday, and the MPV's been driven about 120-150 miles since, with numerous cycles.

Mine had just passed inspection, so I don't have to get it done again, but no CEL, so I'm happy!

[steve12955]

Hello,

Update: the PO421 code hasn't reappeared in over a month and 1,500 miles. The non-foulers work!

Good luck.

Steve

[Rufus]

Just another success story to report: Car failed inspection after recurring p0421 codes. Installed these little guys and after a week of driving, had it retested and PASSED!!!

What do these things actually do to make the code go away??