P0420 Code

I have a 2000 Mercury Mystique comes up on the OBD II code p0420 Catalyst system efficiency for bank one is below threshold. I just had the convertor replaced two weeks ago, and the check engine light came on
yesterday. Anyone know if it could be the 02 sensor behind the convertor?? PLEASE HELP!!! I am getting about 100 miles to a tank of gas........
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If you are getting 100 miles per tank, there is something much more serious than the sensor behind the converter. Not familiar with Contour, but on all OBDII vehicles that sensor is just a monitor, and does not control anything. Are you saying that the only code present is P0420, or you have not rescanned it after the converter replacement?

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The p0420 code came up before we had the convertor replaced, light stayed off for about 2 weeks and the check engine light came back on yesterday hooked it up to the OBD II and it came up the same code. The convertor is new, so I dont understand why it came back up. It just started getting bad gas mileage about 2 days ago.
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mama0205 wrote:

Wow. I think there's a fair chance you've fried the new cat.
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Wow. I think there's a fair chance you've fried the new cat.
See I thought that too, but when I took it back to the shop they said no. And that more than likely it was the 02 sensor. I wanna know why all of the sudden it started getting very very bad gas mileage, when the cat went out it was getting bad gas mileage but not as bad as it is now. When the cat got replaced it started getting good gas mileage up until two days ago, then the light came back on yesterday, makes no sense to me.
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On Sat, 27 May 2006 19:20:46 -0400, "mama0205"

Data stream the O2 sensors to see what they are doing. The front sensor should clock regularly, and the back sensor should not follow it. I suspect you will find the front sensor not clocking properly, and the rear sensor clamped rich. Is this a 2.0, or a 2.5?
***
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Data stream the O2 sensors to see what they are doing. The front sensor should clock regularly, and the back sensor should not follow it. I suspect you will find the front sensor not clocking properly, and the rear sensor clamped rich. Is this a 2.0, or a 2.5?
It is a 2.5. Would that be why I am getting bad gas mileage. I have never owned one of these and have only had it a month. Thanks for all of you help!
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On Sat, 27 May 2006 20:42:28 -0400, "mama0205"

I own a '96 2.5. It's a great little car, and to get as little as 100 miles on a tank I'd have to have my foot to the floor constantly. I did have bad intake manifold gaskets - which gave a lean limit code, and a split hose on the PCV system as well. I did also have to replace the front #1 O2 sensor because it was intermittent - but it threw a code (can't remember which one) indicating the sensor was bad.
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Thanks! We are going to pull the spark plugs out tomorrow and check them out (dunno why) and put a new fuel filter on it.
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thanks for everyones input, however I know now that my car wont get fixed.
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On Sun, 28 May 2006 13:36:44 -0400, "mama0205"

What did you find was wrong with it??
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The other cat is out on the car. He thought that is what it was, called Friday about it, 500 dollars plus you have to find someone to fix it, because the shop in town doesnt do the ones that are behind the head?? I have no idea. Talked to the guy who owns the car said we could trade it in on something else.
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On Sat, 27 May 2006 17:23:15 -0400, "mama0205"

I hate to jump in on this thread 15 months late, but I just had a P0420 code on a 97 CV. Took it to my mechanic, he sprayed the MAF sensor with carb cleaner, reinstalled, and reset the code. It hasn't re-appeared, and I'm getting noticably better mileage. And he didn't charge me a dime.
FWIW.
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