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Discussion Starter #1
So by the title you can see I bought an 07 Taurus. I paid $1100 for 113k miles and took it to a repair shop for an estimate. Apparently the cat is bad which I knew, but the codes say cylinders 4,5, and 6 are misfiring. Also the car is saying it's running rich and lean at the same time. Also, the car idles very rough, jumping rpms randomly. I was told it was probably the pcm by the mechanic and a family friend said the cat can cause a lot if those issues, does anyone know what's wrong?
 

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Break the honeycombs out of the cats and get some "MIL eliminators" (they look like old time spark plug antifoulers) to eliminate 430/420 codes. Check the coil pack VERY carefully for ANY cracks, especially on the bottom. If any are found, replace the coil pack with Motorcraft or BWD only.

A set of OEM spark plugs and plug wires might be in order also.

Cats a VERY sensitive to missfires and will become damaged very quickly if a missfire is present. In other words missfires likely killed the cats.
 

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My first thought is that considering you have misfire on 4,5, and 6, it is happening due to a very clogged cat on that bank (bank 2- code P0430). The back pressure is causing that bank to misfire and run rich, while the PCM tries to compensate by leaning the system out, causing bank 1 to go lean.

We can narrow it down better if you have a live data scan tool. There's many quality ones for cheap nowadays and you can always get advice when it comes time to use it. In fact, sharing just the trouble codes might all but confirm this theory.

Edit: oh and take Jeff's advice above and replace plugs and wires, and preferably even the coil pack if you're gonna go this far. One of those 3 items is likely what started the mess.
 

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Discussion Starter #4
I had the family friend tell me the same exact thing you said Nate, I called a shop to drop it off on Monday and let them know it's probably the cat but they'll know for sure. I really appreciate the help and will update you when they tell me.
 

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Also, normally a clogged cat will bog the engine down and cause power loss, but being a V style engine with 2 banks, if one side is good and the other is clogged, I could imagine this causing your RPM erratic fluctuations.

Please share any updates if you don't need more help; it helps people looking for info.
 

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Update, So I dropped the car off at the mechanic Monday morning. I was called yesterday morning and was told my O2 sensors are bad, the manifold gasket is bad and one other thing was bad that I unfortunately can't remember. But apparently the cat show no signs of being bad or damaged. But, they're confident that if those issues are resolved, the car should run and idle just fine. I won't know for sure until I get the car back later in the week.
 
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