I know it's either the cat or the sensor, but things are telling me that it is the cat indeed, here's the thing:

Cat problem indication:

Low fuel economy and power----Did noticed this when the CEL first initiated. Notice of faster fuel consumption and notice of low power on the car.

Driver’s seat vibrating-----Other’s driver with the same car also has the same problem, and found out the cat is causing this problem.

Cat clogged-----It will notice of low fuel economy and low power, which I did noticed.

OH also, When I first start m car in the morning, I can smell the gas for about a min or so..

 

because the code was heater circuit malfunction or something like that, i posted the code and everybody told me it was the o2 sensor, so i ordered it and the cel is off. where in cali are you, so cal.

 

Middle Ca, in the middle of SF and CA, about three hours drive to each......

 

All those things can be cuased by a non functioning 02 sensor.. The 02 sensors are part of what keeps the engine in check for how much fuel/air to use and will give you bad gas mileage, lack of pwer, etc. I had all these same symptoms on my maxima 3.0 v6 with 117k miles on the original cat, replaced the 02 sensors and BAM! Better mileage/power etc.

 

Ok, my friend, I'm leaning toward to your advice, and listening to you, so if I'm going to replace the O2 sensor, which side should I replace? Left or Right, and which one should I replace, before or after the cat? Do I need to replaced them on both side or just one side?

 

do the sensor first, if that isn't it which i'm pretty sure it is, it would only cost $ 150.00, better than $1000.

 

Please see my previous post.

 

Like I said.. I think Bank 1 (which is what the code refers to) is the left 3 cylinders when you are looking at the engine from in front of the car (someone needs to verify this though, im not 100%).. And it will be the first 02 sensor leading down the exhaust manifold on the catylitic converter. I dont know if you can see it from the top of the engine bay, it might be hidden underneath. Just find out which side of the engine is bank 1, and tell whoever is putting the sensor in that its the sensor on that side before the 3 way cat.

 

"Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter)."

I think left side is pretty accurate guess, and the quote, is this it, the sensor after the cat? on the left side?

 

It should be located down there somewhere if that side of the engine is bank 1.

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The sensor is IN the cat on the left side. 3 cats total.. one on each bank, then further down there is one that joins the 2 together, and then after that is your exhaust piping which leads to your muffler, basicaly.
I could be wrong, maybe there is only one cat which is a 3 way and the 02 sensor is just on the left side of it... My Bentley isnt working right now and it wouldnt surprise me if i am wrong at this point.