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I have a 98 Mercury Sable with the duratec engine, bought the car used about 2 year ago with 125k miles on it, it now has 140k. A couple of weeks ago, the check engine light came on and it scanned to be code P0430 which says: Catalyst System Efficiency below threshold (bank 2). Now the guys at autozone told me it was a bad catalytic converter, as detected by the oxygen sensor on bank 2. Could it be the O2 sensor? I know everyone always says when the O2 sensor goes out, gas mileage decreases. Well ever since the light came on, my car has gotten significantly better gas mileage, over 50 miles more per tank. Its getting almost better now in the winter than in the summer, which from my experience is usually the other way around. Is it possible that when i got my car, the O2 sensor was somewhat bad in that it called for too much fuel when it really didnt need it and now because of the engine light, the system has reverted to some default or loop setting that actually is giving me better gas mileage? A sidenote, we figured a few months ago that the car ran rich when we changed the spark plugs b/c we had to pull the intake manifold off and discovered a lot of carbon build up in there, which we cleaned out. Anyone with any experience with a similar problem? I dont want to do anything to my car that would bring the gas mileage back down, but I dont want that engine light always on either. Is there any way to keep that specific code from showing up but ignoring the problem because in my mind it isnt a problem, its as if before there was a malfunction and now in the default loop it is right. Thanks
 

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A sidenote, we figured a few months ago that the car ran rich when we changed the spark plugs b/c we had to pull the intake manifold off and discovered a lot of carbon build up in there, which we cleaned out.[/b]
I dont think you were necessarily running rich just because there was carbon buildup in there, carbon buildup is normal, especially with that many miles.

Also, there is a bench test for an oxygen sensor to determine if it is working properly or not. Someone will probably post it, if not, PM me and ill be able to find it. The cats shouldnt just up and go bad, though.
 

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More often than not, a P0430 from a Sable or Taurus means a bad catalytic converter. Making sure the 02 sensor is working (i.e., "switching") is a good idea, since it will confirm this. Running rich is the quickest way to kill a cat.

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hey, I have the same thing with my SVT Contour, although I had P0420 which is on the other bank.

After I had the code read at AutoZone, I had the car scanned and tested at my friend's shop. We tested everything, even the emissions. The emissions turned out to be fine, but the 02 sensor was sending strange readings. My car only has 50K miles. So we determined that there was an unusual reading but we couldn't determine the cause. P0420 and P0430 often points to the ineffeciency of the catalytic convertors.

We reset the computer. This code can only be cleared by a scanner, unhooking the battery will not clear the code.

Since then I have not had any problems and I've driven about 500 miles. We'll see if it comes back...

Here is a way you can test the 02 sensors versus the catalysts. The P0430/P0420 code refer to the 02 sensors after the catalysts. What you can do is swap the rear 02 sensors from one bank to the other. If your check engine light comes back on and the code is P0420 rather than P0430, then you know it is the sensor and not the catalyst.


Hope that helps.
 

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Thanks for the responses. Yeah it sounds like that code usually means the catalytic, but I am going to try the testing procedure on the O2 sensors first. The only thing in my case that seems unusual about the problem is the actual increase in gas mileage, why would just the catalytic going bad increase my gas mileage? Also, there are no emission tests in my area so it really doesn't matter from that standpoint to me and I dont want to spend big bucks to replace the cat. If it turns out to be the catalytic, is there any way to somehow have the computer ignore that trouble code so I can know if any other problem ever arises in the future, otherwise my check engine light always stays on. Thanks
 

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My 97 Vulcan threw that code, my mechanic...the guys at autozone...people i knew...said it was a bad cat. I changed the bank 2 O2 sensor after reading some posts around this forum. After changing the sensor, I reset the engine codes and hasn't come back yet. It's been approx 8,000 miles.
 

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P0420 and P0430 codes are NOT O2 sensor problems.

An engine that is running rich will quickly damage the cats.

Steve
 
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