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Thread: The non-fouler trick to fix P0421, for noobs, by a noob

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    The non-fouler trick to fix P0421, for noobs, by a noob

    There are several excellent posts about the non-fouler trick here on these forums. However, they require a basic knowledge of exhaust systems that I didn't have until I had to do it. While I was able to figure it out, I would have liked to have a much simpler guide available. This post is for people like me:

    See this post for some reference pictures:

    http://mazdas247.com/vbb222/showthread.php?t=123634664

    First off, I have a stock 2002 Mazda P5. Other P5s might be different.

    P0421 is caused by the catalytic converter (cat) not cleaning the exhaust well enough. The proper way to fix it is to replace the cat, at a cost of hundreds of dollars. However, we're going to cheat and trick the sensors into thinking that the cat is working correctly, at a cost of less than $20.

    The problematic sensor is the second oxygen sensor. There are two oxygen sensors in your car. Here is a picture of one of them, one taken from further away to help you find it:





    This is the top oxygen sensor. It's easily visible, front and center in the engine. Take a good look at it and get an idea of what it looks like. Unfortunately, this is not the one we want to modify. We want the bottom oxygen sensor, which is underneath the car.

    This picture:



    ...was taken from under the front bumper, looking towards the back of the car. The oxygen sensor is between the wheels, close to the center. In that picture, I've already attached the non-foulers.

    While doing so will make this job easier, you don't need to lift your car for this repair. The sensor is close enough to the front of the car that it is possible to do everything from the front while the car is parked on a flat surface.

    The first thing to do is to prepare the non-foulers. Drill out one of them as per the directions in Rac3rX's post.

    He doesn't make this clear, so I will: when you're done drilling, the piece will be VERY FREAKING HOT. You might say to yourself, "I'll just pick it up gingerly; as long as I don't hold it too tightly or for too long, I should be OK." WRONG. Once you're finished drilling, just leave it be for about 10 minutes while it cools off.

    On that note, you'll be working around the engine and exhaust of your car, so you should only do this repair after your car has been sitting undriven for at least two hours, otherwise you'll be touching very hot engine parts.

    Put the non-foulers together into a single piece as per the instructions in Rac3rX's post. Once you've put your non-foulers together, the next step is to gain access the oxygen sensor:

    There are two protective covers underneath the car that we need to move out of the way. The first one is plasticy rubber and is held in place by a few screws. You won't be able to easily remove it completely, but you will be able to remove a few of the screws and bend it out of the way.

    The second one is metal and is held in place by three screws. It's marked on the picture of the lower O2 sensor. It's a simple protective cover for the actual exhaust pipe.

    At this point, you should have full access to the oxygen sensor behind the metal cover that you just removed. Put some penetrating oil on it so that you can unscrew it later.

    Next, we need to unplug the oxygen sensor and prepare it for removal:

    Use this picture as a reference:



    1. Disconnect the negative battery cable
    2. Find the plug that the oxygen sensor is plugged in to. It's near the top oxygen sensor. See the picture above. The stuff in that picture is more or less directly in front of the driver, pretty close to the front of the car.
    3. There are two plugs there, one for each sensor. The one that doesn't go right to the top sensor is the one that goes to the bottom sensor.
    4. The oxygen sensor plug is attached to the frame. See that little plastic clip near it? There's another one behind it that you'll need to disconnect. Reach behind it and pinch it while jiggling it; you should get it loose fairly easily.
    5. Press your finger to unclip and pull the pieces apart. They've been stuck together for years, and so won't come apart easily.
    6. Tie a string around the plug. We're going to be dropping it down through the engine onto the ground, and a string will make it easier to pull this end back up.
    7. There is a little clamp holding the two sensor cables together. You can open it by pinching it.


    From the bottom, move your hand up the cable. There are two little coils of wire holding it in place. (Careful, they're sharp!) You can remove the cable from the coils without special tools, just twirl it through the coil. Both of these can be done one-handed from under the car, or you can do the upper coil from above.

    When you do that, the oxygen cable should fall down to the ground. You might need to help it. It needs to be loose so that you don't twist the wires while removing and replacing the sensor.

    Get a short wrench and remove the sensor. It should screw right out. Make sure the entire cable twists with it as you unscrew it.

    Once it's out, attach the non-foulers to the sensor. Finally, screw the whole assembly back into the exhaust pipe, making sure that it's on nice and tight. Again, make sure that the entire cable assembly twists with it, so that you're not twisting the wires. Tighten it completely before moving on to the next step.

    Pull the cable back up through the engine. Re-thread it through those wire coils. Plug it back in and reattach the plug to the frame. Replace the clamp holding the two oxygen cables together. Untie the string that you tied to it.

    Get back under the car and replace the protective covers.

    Reconnect your battery, cross your fingers, and start your engine.

    With a little bit of luck, you've just fooled your engine into thinking that your catalytic converter is working normally. Your emissions are higher than normal, and your fuel efficiency is a bit lower than normal, but you will pass your state-mandated emissions. Unless, of course, somebody who knows what they're doing looks under the car and sees the non-foulers.
    Attached Images Attached Images
    Last edited by Shawaron; 07-24-2010 at 11:34 PM.

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    Great howto (it would be nice if a mod moved it to the HowTo section), but the pictures aren't showing up.

    Quote Originally Posted by Shawaron View Post
    Your emissions are higher than normal, and your fuel efficiency is a bit lower than normal, but you will pass your state-mandated emissions.
    This isn't entirely true, you might still pass a proper emissions test (the one where they put a sensor in your tailpipe) if the car has been warmed up. Your second catalytic converter might still be good enough, you'll just be polluting more on cold starts (like on most older cars with one cat).

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    Hmm... The attachments seem to have vanished. I've tried re-attaching them.

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    Registered Member 808mazdap5's Avatar


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    i can see the pictures. might need to update your guys stuff

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    Shawaron - Thanks for posting this. I just completed it and so far no CEL!!


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    My car came with no pre-cat. This trick worked phenomenally, no code since I installed it. Awesome write up!

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    Registered Member rit14623's Avatar

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    Note, don't get the Bosch O2 sensor. The tip is too wide for the non-fouler.

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    Update: The CEL came on the next morning when immediately upon starting my car. I'm not sure why it's not working, I did everything correct. But it sounds like the things to check are, 1) Is there a exhaust leak 2) Was the hole drilled big enough. It did feel like I had to really cram the sensor in the non-fouler to get the threads to connect. Could this be the problem? Not enough room in the non-fouler? Thanks.

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    The Diagram Dude pcb's Avatar

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    Did it throw the P0421 code ??

    The tight fit may be blocking the sensor too much. It need s to "sniff" just a bit of exhaust.

    I don't know how the ECU defines "two drive cycles" but I would think the CEL would come on once the car is warmed up. (unless it waits for "engine off" to define a drive cycle)


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    Haha love the title to this thread, well from another noobs experience from doing this... You need to make sure that you use very strong drill bits, I think the carbon drill bits, I remember doing this at my jobs warehouse and I picked a black bit thinking it would do it and I prob drilled that thing for like three hours till I got it to fit the O2 sensor. Its stainless steel I think and youll prob need some gearing oil to drip in there while drilling it out. But trust me go to home depot and get a good strong drill bit, not sure which one is the best, and you might want something that can hold it tight for you while drilling. But this fouler trick is flawless, I was so happy when I got it in.

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    The Diagram Dude pcb's Avatar

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    I decided to try and drill out my non-fouler and you're right,.. that's pretty damn hard metal,.. I found this youtube video about how to drill stainless,... I'm gonna give it a go with my cheap ass but still sharp drill bit.

    http://www.youtube.com/watch?v=tF9-lqBKBOQ

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    Quote Originally Posted by pcb View Post
    Did it throw the P0421 code ??

    The tight fit may be blocking the sensor too much. It need s to "sniff" just a bit of exhaust.

    I don't know how the ECU defines "two drive cycles" but I would think the CEL would come on once the car is warmed up. (unless it waits for "engine off" to define a drive cycle)]

    Thanks for this info, I'll trying drilling the hole bigger and see if that fixes it.

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    The Diagram Dude pcb's Avatar

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    It worked awesome,... even with a chewed up drill bit.

    I plugged the small hole with grease then filled it up with 5W30 then drilled it out in less than 45 seconds.

    I don't have a 1/2" drill bit so I used my 7/16",... my 5/8" looked too big,.. gotta go shopping.




    My Bosch drill works well because you can lock the trigger at half the speed.

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    The Diagram Dude pcb's Avatar

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    Quote Originally Posted by PortlandChris View Post
    Thanks for this info, I'll trying drilling the hole bigger and see if that fixes it.
    Did you check your code ??? You wanna make sure you're not fixing something that ain't broke. There is a bunch of other codes related to the rear sensor.

    You might have a bad wire or something.


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    Quote Originally Posted by pcb View Post
    Did you check your code ??? You wanna make sure you're not fixing something that ain't broke. There is a bunch of other codes related to the rear sensor.

    You might have a bad wire or something.
    I ran the codes and it's showing P0140 and P0037, so circuit low and no activity detected. Could this be caused by the tight fit in the non-fouler?

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