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Car Forum / Ford / Ford Cars / August 2007

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P0420 Code

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mama0205 - 27 May 2006 22:23 GMT
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........
Happy Traveler - 27 May 2006 22:57 GMT
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?

> 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........
mama0205 - 27 May 2006 23:20 GMT
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.
zwsdotcom@gmail.com - 28 May 2006 00:06 GMT
> 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

Wow. I think there's a fair chance you've fried the new cat.
mama0205 - 28 May 2006 00:20 GMT
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.
clare at snyder.on.ca - 28 May 2006 00:57 GMT
>Wow. I think there's a fair chance you've fried the new cat.
>
[quoted text clipped - 4 lines]
>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.

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?
mama0205 - 28 May 2006 01:42 GMT
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!
clare at snyder.on.ca - 28 May 2006 04:29 GMT
>Data stream the O2 sensors to see what they are doing. The front
>sensor should clock regularly, and the back sensor should not follow
[quoted text clipped - 4 lines]
>never owned one of these and have only had it a month.  Thanks for all of
>you help!

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.
mama0205 - 28 May 2006 07:08 GMT
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.
mama0205 - 28 May 2006 18:36 GMT
thanks for everyones input, however I know now that my car wont get fixed.
clare at snyder.on.ca - 28 May 2006 20:25 GMT
>thanks for everyones input, however I know now that my car wont get fixed.

What did you find was wrong with it??
mama0205 - 28 May 2006 20:38 GMT
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.
Tim J. - 29 Aug 2007 07:36 GMT
>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........

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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