I went to the AutoZone this evening and they pulled up the code right in front of me. Only one code came out. P421. The guy right away said its O2 sensor (on the front bank). He was little confused though about the number of sensors. He showed me the sensor on the front bank ( and underneath the car in the middle ) but we both couldn’t see any sensor on the other side. He said there should be three O2 sensors . I asked him about the catalytic converter. He said that he is 99% sure its O2 sensor because if converter goes bad, fuel efficiency would be horrible and other problems may also occur like car wont start etc. I don’t know how much he was right in his analysis but I am very much looking forward to changing the sensor with a hope it would solve the problem. Babyhuey and kennin , I am highly indebt to you guys for the help . Without your guidance I would have spent a lot of money.
Regards

 

there are 4 sensors, 2 pre cat, and 2 post cat. the cats i refer to are sometimes called pre-cats, they are part of the exh manifold. i aint good at this, so this might not work, but here is a link with a picture of a pre cat:

http://www.racepages.com/catalytic/mazda/millenia.html

scroll down, and look at the eastern one. where the exh exits the manifold, it gets really big. that is the pre cat, there is a sensor before it, and right after it. there are 2 pre cats on the car, and then go into a "Y" joint, become 1 pipe, and then there is a main cat, then out the rear to the muffler.
while it still could be a bad cat, my money is on the oxygen sensor. here is a link(again, hope it works) to where i get my mazda parts from:

http://www.trademotion.com/partlocat...&siteid=214284

pre cat: KJ01-18-861A-9U
post cat: KJ03-18-861B-9U
input the numbers with no dashes, not case sensitive.
these are the ones on the front bank

i have yet to change the sensors on either of our cars, but looking at them, i know at least 3 of them are easily accessible from underneath. also, then rascals are usually ON there, and there WILL be some cussing needed, and special tools to remove them. it sometimes helps to run the car and get the cats hot, then break them loose. i find that a torch makes me feel better, cause i like getting even.
you can also get generic sensors for about 50 bucks each, you have to reuse the old harness, and care is needed, there are 4 wires, and if they get crossed, you might fry the ecu. this will also take more time, and if a shop is doing it, they wont want to go this route. dont even bother with getting used sensors.

last but not least, if you do get new sensors, not generics, i would like to buy your old ones. i do have generics, and i can use the harness to connect them, then it is way faster to just switch them out.

 

Hi kennin,
I gave call to autozone guys and they said that universal sensor is the same as the OEM except the joining hook ( as you described ) . but heres my real question. with P421 Code. Do i need to replace all the four sensors or the one on the front bank as said by autozone guy? ( plus its 2002 millenia S, miller, what size of engine it has, 2.3 or 2.5? )If its the only one sensor ( or even two) I would go with OEM part and you dont need to buy it . I will simply ship the old ones to you ( just to return a little portion of the help I have got from you and babyhuey). Thanks

 

there are 2 sensors on the front bank. the part numbers i gave you should be correct, perhaps babyhuey can confirm. however, you must realize that the cat still COULD be bad. usually you start with the after the pre cat sensor, then the before. if it was me, i would just go ahead and change both. if the code comes back, then the cat would be suspect. however, if you were to change the cat, you would need to change the sensors, too, so it would not be a waste of money.
also, shops dont like people to bring them parts to install. they make money on parts and labor. if you are going to get your own parts, you should ask first about it. most shops wont get the price discount on genuine parts like you can from the link i posted, because they cant wait for the part to come in. they will get it from a dealer, or another supplier who has it in stock, and it may cost more. here is another link to aftermarket sensors:

http://www.rockauto.com/catalog/cata...&parttype=5132

there are some ac delco sensors that are plug n play for about 81 bucks, not a bad price. i believe the original equipment ones are made by denso, and the delco ones could be denso's that they get and rebox. the universal ones are the ones that need to be spliced it to the original harness.

one last word of advice. shops dont like to be told they are wrong, especially by customers. i wouldnt take the car back to any of those places that told you the car needs cats. you might want to start fresh, and find a shop who will scan the codes, perhaps then you could just casually mention that there was a tsb for earlier cars about the 0421/31 codes being an oxygen sensor. see what they say. look for shops that are clean and well organized, and have been there for a long time. dont expect to find a place that has a lot of experience on the millenia. if you do, you are lucky.

 

Whoa there you happy posters.

I think we better back up just a few notches here. The Autozone guy told you it was a code P0421 and was the front bank O2? That would be a negative. The 421 is right bank cat efficiency. Right bank is the one on back of engine bay. left bank is near radiator and would be a P0431. Good idea to use the OE. Part numbers from kennin are good for OE left bank O2's but we need right bank rear. The OE numbers out of the parts listing will not be valid as you will just be reinistalling another 'overly sensitive' O2 per the TSB. The part number for the right rear update O2 is KJY2-18-861. Msrp is $133.80. Can get from www.onlinemazdaparts.com for $107.04.
The Milly S is the 2.3l engine somali. Will try to include here the entire text of TSB below


TSB: 01-003/01
MIL ON - DTCS P0420 P0421, P0430 P0431
APPLICABLE MODEL(S)/VINS
All 1997-2000 Millenia S.

DESCRIPTION
Some vehicles may exhibit MIL illumination with DTC(s) P0420/0421 or P0430/0431 stored in memory with no driveability concerns present. This condition may be caused by an overly sensitive rear oxygen sensor(s), which falsely indicates an emissions concern and a DTC to be stored.

Customers having this concern should have their vehicle repaired using the following procedure.

REPAIR PROCEDURE
1. Verify customer concern.
2. Confirm FFD and stored DTC(s).
If DTCs P0420/0421 or P0430/0431 are indicated, proceed to STEP 4.

If any DTCs are indicated other than P0420/0421 or P0430/0431, perform the appropriate repair for those DTC(s) according to the appropriate troubleshooting section of the Workshop Manual and proceed to STEP 3.

3. Reconfirm FFD and stored DTC(s).
If DTCs P0420/0421 or P0430/0431 are now present, proceed to STEP 4.

If no DTC(s) are present, return vehicle to customer.

4. Replace rear O2 sensor only on the side that the MIL and DTC indicates. Refer to EXHAUST SYSTEM REMOVAL/INSTALLATION [KJ] in the Workshop Manual section 01-15B.
NOTE:

DO NOT replace both left and right side O2 sensors at the same time unless stored DTC(s) indicate both sides have concern.

DTC(s) P0420/0421 indicates right side O2 sensor. DTC(s) P0430/0431 indicates left side (closest to radiator) O2 sensor.


5. Confirm the switching ratio of the W-U TWC is greater than 2 to 1, by using the NGS tester and monitoring according to 2000 Workshop Manual section 01-16B.
If inversion ratio is greater than 2 to 1, repair is completed, proceed to STEP 7.

If inversion ratio is less than 2 to 1, proceed to STEP 6.

6. Replace W-U TWC according to EXHAUST SYSTEM REMOVAL/INSTALLATION KJ in the Workshop Manual section 01-15B.
7. Verify repair.
PART(S) INFORMATION


Part Number
Description
Qty.
Notes

KJY1-18-861
Rear O2 Sensor
1
Left Bank

KJY2-18-861
Rear O2 Sensor
1
Right Bank

 

great info. i just saw front, so i went with that. just goes to show how easy it is to screw up. also, thanx for the updated part numbers. the tsb dont cover his car, tho, is it possible that the updated sensors are already on the car, or were they only for the 97-00 my cars?
also, i have read a couple of posts were the rear(after cat) sensor was changed, and the car would still throw the code. changing out the pre(before) cat sensor seemed to work. for me, since it looks like overlapping labor, when i do it, i will just change out both. also, there is a plug that some of the mustang owners use, it is like and extender, it moves the post sensor farther out, so it is not as sensitive. iirc, one poster did that, and it seemed to have worked.

it's great to have a real mazda tech to answer questions.

 

I will give call to local shops next week. I think someone should be able to put them on if I provide the parts. In summary, I need two O2 sensors on the right bank ( back side of the engine ) and I should replace both sensors to cut the labor cost .
I will keep you guys updated. By the way , all the information that has been provided so far is for1995-2000. I am assuming that O2 sensors and catalytic converters (plus the over all exhaust setup) are the same for all millenias from 19xx-20xx. Thanks

 

Just a quick note. I have driven this car for around 200 miles after autozone guy reset the computer. No check enginle light so far. It used to come after 30 or maximum 100 miles. Thanks

 

that is a good indicator it is the oxygen sensor. it is dying, you will have to change it soon. get yer wallet ready. look around for a good shop. buy a scan tool.

 

Indeed! I am looking around for a good shop. I have found one mechanic who is ready to install the sensors. I will keep you guys updated. Thanks a lot for the help. You guys really saved me from spending hell amount of money.