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-   -   Catalytic converter or Oxygen Sensor (http://forums.mazdaworld.org/100-millenia/53140-catalytic-converter-oxygen-sensor.html)

mazda-fla 11-02-2004 04:17 PM

See this post for changing the O2 sensors.

http://forums.mazdaworld.org/index.php?showtopic=9886

The problem is, that since you get both codes P0421 & P0431, that there is a fairly good chance that your pre-cats are bad.

If the light has been on for over a year, how did you pass state emissions last time? How close were your to failure?

Sometimes, you can pass emissions with a bad cat. It just depends on how bad your cats have deteriorated. Because you also have a main cat downstream from the pre-cats that help reduce pollutants. So it is possible to have bad pre-cats and a CEL, but still pass emissions. (As long as your state just tests emissions by putting a probe into the tailpipe. If they also hook up to the OBDii, then the CEL would cause failure in that case).

If all you care about is passing emissions, and the cats turn out to be just good enough for that, you could install an O2 sensor simulator from ebay for about 12 bucks that would transmit signal to the computer that the emissions from the pre-cats are within specs (2-to-1 inversion ration), and that would make the CEL turn off. But driving around long-term with bad, plugged-up, cats will ultimately ruin your car.

It's kind of open-ended trying to fix those codes yourself. But, if it were me, I would tend to favor replacing both Left and Right Rear O2 sensors first. Then, I might even go so far as replacing both Left and Right Front O2 sensors second. Then, if that doesn't work, you are probably looking at bad pre-cats. Then things start getting expensive.

Waveoner76 11-02-2004 05:07 PM

Thanks again.. I'll post the outcome of my screw up of not changing the sensor 1+ year ago.

Waveoner76 11-10-2004 05:03 PM

K.. I've decided to change the 2 rear o2 sensors. After looking @ your tech guide (which is very good), is there any way you can tell me EXACTLY where the 2 rear o2 sensors are. I'm going to try to change myself with the generic brand you bought from the web site.

Thanks

devilbrad 11-10-2004 05:49 PM

Quote:

Originally posted by Waveoner76@Nov 10 2004, 07:03 PM
K..* I've decided to change the 2 rear o2 sensors.* After looking @ your tech guide (which is very good), is there any way you can tell me EXACTLY where the 2 rear o2 sensors are.* I'm going to try to change myself with the generic brand you bought from the web site.

Thanks
[snapback]217954[/snapback]

Hate to throw a wrench into this, but there are different codes for the rear (and front) O2 sensors. One code can say the sensor is inactive, one can say the heater for the sensor is bad, the third (in the case of the downstream O2 sensor, after the precats) will say the cats bad. Unfortunately, yours are the codes saying the cats are bad. Changing the O2's wont fix it, the new O2's will read the same inversion ratios as the old ones and throw the code again. More research into the meaning of the specific codes can prevent unwanted spending of money. Also remember, the rear O2 sensors have NO EFFECT on the air fuel ratio of vehicle, all they do is monitor the efficiency of the precats. So almost everytime you get a rear O2 code, its cause the cat is bad. One other thing to check, which is kinda difficult on the Miller Cycles, but very common......some of the plastic vacuum tee's under the intake manifold will crack and break, creating vacuum leaks, this will damage cats. I do them all the time at work. If you have any questions, feel free to PM me, I'm a Mazda master tech, been working on em for close to 10 years now.

ho chi inn 11-10-2004 06:07 PM

Quote:

Originally posted by devilbrad@Nov 10 2004, 06:49 PM
Hate to throw a wrench into this, but there are different codes for the rear (and front) O2 sensors.* One code can say the sensor is inactive, one can say the heater for the sensor is bad, the third (in the case of the downstream O2 sensor, after the precats) will say the cats bad. Unfortunately, yours are the codes saying the cats are bad.* Changing the O2's wont fix it, the new O2's will read the same inversion ratios as the old ones and throw the code again.* More research into the meaning of the specific codes can prevent unwanted spending of money.* Also remember, the rear O2 sensors have NO EFFECT on the air fuel ratio of vehicle, all they do is monitor the efficiency of the precats.* So almost everytime you get a rear O2 code, its cause the cat is bad.* One other thing to check, which is kinda difficult on the Miller Cycles, but very common......some of the plastic vacuum tee's under the intake manifold will crack and break, creating vacuum leaks, this will damage cats.* I do them all the time at work.* If you have any questions, feel free to PM me, I'm a Mazda master tech, been working on em for close to 10 years now.
[snapback]217964[/snapback]

this is a huge problem. is there any way you can give us diagrams on which tee's, or where abouts they is?

mazda-fla 11-11-2004 09:36 AM

Quote:

Originally posted by devilbrad@Nov 10 2004, 06:49 PM
Hate to throw a wrench into this, but there are different codes for the rear (and front) O2 sensors.* One code can say the sensor is inactive, one can say the heater for the sensor is bad, the third (in the case of the downstream O2 sensor, after the precats) will say the cats bad. Unfortunately, yours are the codes saying the cats are bad.** Changing the O2's wont fix it, the new O2's will read the same inversion ratios as the old ones and throw the code again.* More research into the meaning of the specific codes can prevent unwanted spending of money.** Also remember, the rear O2 sensors have NO EFFECT on the air fuel ratio of vehicle, all they do is monitor the efficiency of the precats.* So almost everytime you get a rear O2 code, its cause the cat is bad.* One other thing to check, which is kinda difficult on the Miller Cycles, but very common......some of the plastic vacuum tee's under the intake manifold will crack and break, creating vacuum leaks, this will damage cats.* I do them all the time at work.* If you have any questions, feel free to PM me, I'm a Mazda master tech, been working on em for close to 10 years now.
[snapback]217964[/snapback]

I am aware that the P0421 & P0431 codes indicate bad pre-cats. However, there is a TSB that indicates replacing rear O2 sensor(s) first. I realize that the cause is most likely bad pre-cats. However, if it was me, I would first spend the 100 bucks and change the rear O2 sensors myself before I spent $2,000 on new pre-cats. There would also be no loss to me if it turned out to be bad pre-cats. Because if it turned out that I had to turn right around and immediately replace the pre-cats, I'd want to have new O2 sensors all around anyway.

Technical Service Bulletin No: 01-003/01

Subject: MIL "On" - DTCS - P0420/P0421, P0430/P0431

Applicable Models: All 1997-2000 Millenia S.

Description: Some vehicles my exhibit MIL illumination with DTC(s) P0420/P0421 or P0430/P0431 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. Replace Rear O2 sensor only on the side that the MIL and DTC indicates.
2. DO NOT replace both left and right side O2 sensors at the same time unless stored DTC(s) indicate both sides have concern.
3. DTC(s) P0420/P0421 indicates right side O2 sensor. DTC(s) P0430/P431 indicates left side (closest to radiator) O2 sensor.
4. After replacing O2 Sensor, confirm the switching ratio of the Warm Up Three Way Cat is greater than 2 to 1, by using the NGS tester. If inversion ratio is greater than 2 to 1, repair is complete. If inversion ratio is less than 2 to 1, then replace Warm Up Three Way Cat.

Parts Information:
KJY1-18-861, Left Rear O2 Sensor. (if code is P0430/P0431)
KJY2-18-861, Right Rear O2 Sensor. (if coe is P0420/P0421)

Waveoner76 12-24-2004 08:49 AM

Well here's an update!! I bought 2 used pre-cats from a fellow MW member that works @ a mazda dealer. One of them came with the rear O2 sensor. I took the ride to a shop, and after looking under the car they didn't want to take the job. They said I should take it to an engine shop. So i'm like OK, just swap out that 1 O2 sensor.

Got the sensor swapped, went to an AutoZone, got the CEL codes cleared. After driving the car for about 225 miles (about 1 week ago), the CEL came back on. 2 days ago, the CEL went off on it's own. :thumbsup: (strange because I had both codes, left & right rear o2/pre-cat but only swapped 1 sensor)

I never got to pull the CEL codes before it went off so I don't know if it was the same codes that came up as before. Hopefully it will stay off. (by the way in case you're wondering, the CEL light bulb did not burn out.)

Does anybody know a good Mazda mechanic in the NorthEast that could handle a job like changing the pre-cats. (Just in case it does come back).

For those of you who contributed with great information & advice, thanks. This site is great.


reality1011 12-24-2004 10:33 AM

yes indeed very good information... do you have the S engine?

Waveoner76 12-24-2004 11:01 AM

yes... 98 milly S w/ about 115k miles.

reality1011 12-24-2004 11:25 AM

hmmm you might be able to get the work done by a non-mazda mechanic... that would save you some $$$ but if it gets too in-depth then you might want to stick with the mazda mechanics


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