I saw the 'front' as well kennin but I am more used to calling them upstream and downstream instead of front and rear. Also use right bank and left bank to differential from side to side.
Is true that the TSB only covers from 1997-2000 but all four sensors have been the same part number from 1995-2002 on Milly S so why would it not apply? The reason is in the last two digits of the TSB 01-003/01. Those digits mean that bulletin was released in 2001 so safe to say that the bulletin is only covering vehicles that might still be under warranty. Why was part number for downstream not changed after the bulletin to the updated O2 part number. It has superceeded to updated part 4 times since 95 anyway.
If you still want to change both O2 sensors for right bank then use KJ02-18-861C-9U for upstream and modified sensor KJY2-18-861 for downstream. Am very doubtfull that it will stop the code but is certainly a cheaper alternative to try if that is your choice somali.
Replacing the front could have a bearing on CAT efficiency but if the output voltage was cycling that slow it would probably set a sensor fault code before an efficiency code.

Perhaps a brief explanation into how an Oxygen sensor, Catalytic converter, and Efficiency works and is measured. Not a great teacher but will try to explain in more laymans terms. If a part does not make sense then tell me and will try to explain better.

First off is How a Catalytic converter operates. The inside of a CAT converter is a honeycomb material made of different rare earth elements that all of the exhaust gases pass thru. With a combination of very high heat and enough oxygen molecules, the CAT converter creates a catalyst condition that combines the oxygen with the polluting elements of combustion by-products to create a cleaner exhaust. The first big polluting element is unburnt fuel or HydroCarbons (molecular formula of HC is one molecule of Hydrogen and one of Carbon). Break that HC apart and you have a free H and a free C. Add two H's and half of an oxygen molecule (O2) and you have H20, otherwise know as water. Add a full oxygen or O2 molucule to the C and you get CO2 or Carbon dioxide which we exhale with every breath. The second big pollutant is Carbon Monoxide or CO. Again, add one Oxygen and you have CO2 or Carbon dioxide again. This oxygen consumption by the CAT converter is also how it is measured to be good or bad. Later on that though.

Now for an Oxygen Sensor. An O2 sensor is exactly what the name implies. It senses or detects the concentration of oxygen and produces a corresponding voltage to tell the engine computer this concentration of oxygen in the exhaust. It is for lack of better words, a battery, as it produces its own voltage between zero and one Volt DC based on these concentrations. Low amount of oxygen in exhaust would result from a rich intake mixture and sensor would have a voltage output between 450 milivolts and 1000 milivolts or 1 volt. The lesser amount of oxygen there is, the higher the voltage. Same for high amount of oxygen in the exhaust due to lean intake mixture. Output voltage for lean is between 450 milivolts and 0 milivolts. The more the amount of oxygen the lower the voltage.
There are two 02 sensors for each bank or side of the engine. One is Upstream of the CAT and other is Downstream of the CAT.

The upstream O2 is used to monitor the air/fuel mixture so that the engine computer can make final calculations about how much fuel to be squirting in the engine. O2 reports lean exhaust with low voltage and computer then richens up the mixture by leaving the fuel injector open a few extra miliseconds. Same for rich exhaust, high voltage from O2 means rich exhaust and computer closes injectors a little quicker. That is the only function of the upstream O2 sensor. Cycling from rich to lean to rich to lean. At higher rpm as many times as 4 per second. Each rich to lean or lean to rich condition is call an INVERSIONas it crosses over the 450 milivolt mark.

Now the downstream O2 operates in the exact same manner in oxygen concentration detection as the upstream. The difference is that this sensor is only used for checking the efficiency of the Catalytic converter. Engine computer does not use signals from downnstream sensor for any mixture corrrections but only looks at what it's voltage is doing when checking the cat converter for being good or bad or marginal. Note that pre-OBDII emissions standard cars before 1995 did not have a downstream O2 as catalyst efficiency was not required before that era. OBDII required carmakers to be able to monitor the condition of the Cat converter and turn on the CEL light with code when condition of CAT converter became deteriorated.

Now that we know how O2 sensors and CAT converters work, How does the PCM or engine computer know when the CAT is not being efficient????
As earlier we know that the CAT converter needs to be very hot to be functioning correctly. The converter actually cools down when you are at an idle. The efficiency check is always performed after engine rpm has been high and steady for at least several minutes and then dropped down to a idle. At the higher rpm, both sensors should be changing from rich to lean quite rapidly due to the volume of exhaust flow. When rpm is dropped down to an idle however, the air flow past the sensors and thru the CAT decreases considerably. At this point the upstream O2 sensor is continuing to change rich, lean, rich, lean but at a slower rate. This is normal.

The downstream O2 is the crucial signal at this time. Keep in mind that a CAT converter consumes oxygen by creating that catalyst reaction to reduce the pollutants. Based on this,
A GOOD catalytic converter should be using the majority of the oxygen in the exhaust so the downstream O2 would have a fairly steady high voltage. .650-.800 milivolts is ideal.
A MARGINAL converter would start to see some INVERSIONS or rich lean signal changes at the downstream O2.
A BAD converter is determined if the number of downstream O2 INVERSIONS are at least half of the number of upstream O2 INVERSIONS or 2:1 ratio. For example, in a 30 second test period the front O2 changed from rich to lean or lean to rich 30 times that would be 30 INVERSIONS. If the downstream O2 during that same period had 15 or more INVERSIONS, then PCM judges that the CAT converter efficiency is not up to standard and turns on the CEL with CAT efficiency code stored.

I know it was a long read but maybe this will shed a bit of light on these mysterious recurring codes.

 

Hi babyhuey,
Thanks for explaining the function(s) of O2 sensors and Catalytic converter. So the feel I am getting from your post is that even if i change the two sensors it wont address the CEL and its pretty much the CAT thats bad. I would highly appreciate if you could enlighten me a little more on this. I thought you and kenin agreed upon having bad O2 sensors.Thanks

 

my position on your cel/problem is that it COULD be the cat(s), but i would change the sensors first. you will need to change the sensors with the cat anyway, and it seems way more sensors are changed than cats. THEN, if you still have problems/codes, you may need a cat. if the cat was bad, the ce lite would come back on right away. sensors dont just die, they start to slow down. the reason a shop/dealer will change them is because the car will probably come back withing a short time with the lite on again. depending on how much you drive. our white car is only driven about 4k/year, i have reset the 0421 code twice in the last year.
keep in mind this is a guess based on my own experience, and what i have read. babyhuey has forgotten more than i know about millenias. most of my experiences on cats and oxygen sensors come from bmw, i worked in a shop for a while, now i sell parts for them.

 

I would not underestimate your knowledge and ability in any way kennin. The fact that you own two of them and do much of your own work gives you plenty of knowledge. Yours is first hand daily driver experience which is invaluable.
My experience thus far has been on the lower mileage cars, say under 30-40k the O2 seems to do the trick. On higher milers the Milly always seems to come back in a month or 6 with code set again and fails testing. If I had 15 minutes with your car somali, and my IDS computer, I would be able to tell you if the CAT was ok, failing,or outright bad just by looking at those inversions on a recorded graph. Since that is not probably going to happen unless you want to make a trip to Oregon it is really up to you. Replacing the O2 sensors will certainly not hurt anything and I usually recommend replacement of the upstream O2 around the 100k mark anyway. Remember that those sensors start at an outside temperature when engine is started and are heated to around 700'F in 10-20 seconds. Cool back off when you shut off and heat up again, and again, and again. That is brutal treatment for anything. It is certainly a less expensive alternative to outright replacement of the warm-up CAT anyway.

 

So I will first change the sensors and if check engine light comes back I will put a new catalytic converter. babyhuey, you are very far else I would have preferred to have you look at the car. I am in iowa[sm=smiley21.gif]. Thanks

 

The sensors are changed (by universal ones). Check engine light went off and after 100 miles, turned on again (like before). Catalytic converter is bad? Now I am thinking why I didn’t buy the OEM sensors. Those autozone guys and even the mechanic recommended universal ones. Both sensors were the same. However, mechanic told me that he had to connect 4 wires on one and 3 wires on the other one (both on the firewall side, right bank). I have also bought the scanner and it still gives me the same code, P421. I am confused now. May be universal sensors are not compatible and if I change the cat, light may turn on again. May be sensors are good, cat is bad. I don't know how to check the inversions of cat. Thanks

 

Am very sorry to hear that the CEL came on again so soon. Am not a big fan of universal O2 sensors but that was the choice so there it is. Is very easy to check the Inversions but only need to have a digital volt meter to look. They are available at radio shack for as little as $20 but are an invaluable tool in checking the electronics of the car from enigne thing to body stuff. I can give you the location of the connector and wire color to look at and what readings to look for and that will give you the Inversion information that you are looking for.

 

babyhuey,
Thank you very much for the reply. Yes i really need your help to figure out whats exactly wrong in this car. I will go and buy volt meter tool .You might need to help me in baby steps because I don't know much about it . I will let u know as soon as i got everything ready. Thanks a lot!

 

wow. sorry to hear about the ce lite, too. i am still not sold on the cat, and you would have needed new sensors anyway. i am interested in learning how to test, too.

 

I have a 2002 Madza 626 with the exact situation like Somali. This message form is simply amazing. Hats-off to you Keninn and Babyhuey. You guys have explained it well.
I got the CE light on a couple of days back. I got an enginedaignose from a known car care service provider and he has recommended to change:
FS1N20500 CATALYTIC CONVERTER[/align]FS1J18861B OXYGEN SENSOR (UPPER) [/align]FS1K18861B OXYGEN SENSOR (LOWER) [/align]the parts are only available from OEM thru dealership and this has got into quite an expensive affair. Tomorrow I plan to go for a second opinion with not so known car care service provider. [/align]I am dying to know how Somali solved her problem.[/align]I am extremely impressed with this forum. Excellent stuff![/align]