How did you come to the conclusion it's the cat?
You mentioned the rear plugs were not changed? But the front ones were? That could be a big part of the issue right there. Or another item that might cause it could be the fuel pressure regulator.
Just a couple of thoughts.

Or you could just temporarily remove the upstream O2. I'd rather not drill-n-fill things if I don't need to.
Another thing you can check, depending on the level of potential cat failure, if the cat is clogged real bad, if it's cherry red while the car is running that's a dead give away as well.

 

the car is having a hard time throttling (like something is holding it from going to a high rpm) when it's hot. And the CEL comes on and off sporadically. I'm going to get a scanner to check the o2 sensor readings. I replaced the rear spark plugs and wires already. I also replace the air filter and thermostat. I took off the throttle position sensor and mass air flow sensor and that didn't solve anything.

 

Sorry, but I'm still stuck on what you said about the rear plugs not sparking. Especially after having recently changed out your wires and plugs. You still haven't said how you came to that conclusion.
If your CAT is clogged it would not prevent your plugs from firing. I think if you find the cause of that problem, you will have found the cause of your sluggishness. When my CAT was clogged it caused poor acceleration, very bad gas mileage and smelled bad too.
With the P0300 code in your CEL, I would follow Jason's (The_Maniac) suggestions and Check your Fuel and Ignition Systems. After all, That is were your DTC is telling you the problem is!

 

This is where I am confused. A clogged cat prevents exhaust gases from leaving the car. In park, no load on the engine, unless you are heavily clogged, idle and some revs should be OK.

The question about the mis-fire is "why". Car needs fuel, spark, air (oxygen to be specific), one of these three is not working right.

If you remove all 6 plugs and only the rears looked bad, the next question would be, what do they have in common? P0300 is also a random mis-fire, so it does not focus on a one cylinder. A good scan tool can give you some data about which cylinders are mis-firing. That data could help here.

I'm not feeling ICM or coils (as some fronts would share in that). And if it was a coil, you could swap them and see if the problem moves. Or push come to shove, if you know someone with a GM 3100, 3400, 3800 from the early 90's to present, you could always trade coils and see if the problem follow the coils.

Assuming my gut is right and it's not ignition/spark, this leaves air and fuel.
If the cat is clogged enough, I suppose the back head would be affected more then the front just by how exhaust gases would back up. Again, as suggested, a simple test would be pop the O2 sensor out of the rear mani and let it just vent (just a short experiment at idle should not harm it). But this would give the exhaust another exit in the event of a clogged cat and should improve, if it is a clogged cat. Again, if it's this bad, that cat should glow red as the car warms up/gets to operating temp. That visual locks in a clogged cat.

Now, let's assume the cat checks out. This leaves us with fuel. If it was one one or two cylinders, we could guess a bad injector or two, a badly seated injector or a bad o-ring on an injector. But 3 out of 6 plugs (all on the back of the engine) raises a question. Looking at the fuel rail, the fuel enters the front of the rail, just before the fuel pressure regulator. If the regulator cannot keep proper pressure, the fuel supply is going to get weaker as the fuel rail gets to the last injector. The front 3 are the first three to get fuel from the rail, the back ones rely on enough pressure to flow fuel up and over the UIM through the fuel rail.

This means either a weak fuel pump. I'll mark that unlikely (not impossible), normally when a GM fuel pump dies, it's dead. Even if it engages, it lacks ability to build up pressure. My next guess is the fuel pressure regulator. One member here removed the vac line on top of the regulator and found the line full of fuel. If this is the case, it's a dead give away that the regulator has failed.

Based on what little I know about this situation, I would focus on the fueling. I would begin with the fuel pressure regulator.

Again, this is based on what I know/understand about your current situation. There may be details unknown or mis-understood.

 

honestly I'm focusing on the cheapest and easiest thing to work on. I believe the cat is cheaper and easier to work on than the fuel regulator. I'm honestly not seeing any signs that's it the fuel regulator. I'm not smelling any gas and there's no black smoke coming out of the exhaust/muffler.

 

I never said the rear spark wasn't sparking, I just said it look like it hasn't been changed in some time. So I changed all the spark plugs and it didn't work. The old spark plugs may have caused the cat to go bad from not changing it.

 

I think you have it backwards. FPR is about $30-$50, easy DIY item. The cat, for just the part will probably exceed $100. Plus, it's probably not going to be a fun driveway change part.

You're missing the point here. One way the FPR failed for a member, it allowed the fuel to get sucked into the connected vac line. But, that is not the ONLY thing it may do when it fails. It may simply be starving the fuel rail of fuel. You won't get additional fuel odors and if you don't have enough fuel, you're certainly not going to get smoke from fuel (which I believe is whit-ish in color, not black).

If they allowed a lot of unburnt fuel to dump down the exhaust, it's possible, but this would have had to be happening for a long time.

Just as an FYI, cats don't fail too often. GM had a couple of years (00 and 01) that they had a run of cats that failed early. They do fail, just not often. A cat is not something I would just throw a part at to see if it solves the problem. Try some diagnostics and see if the problems go away. In addition to what I suggested, I believe an exhaust shop can pressure test your exhaust to determine if the cat is clogged.

 

I have a question, when you put in the new plugs, did the PO300 code go away? If it did, Cool! But if it didn't Why ignore what your ECM is telling you?
Jason, (THE_MANIAC) is one of the most knowledgeable members on MCF. I've been working on cars for 33 years and I will often take his advice as solid! Especially with any year Monte Carlo.
I work on my car in the street around the corner, under a shade tree. My budget is not great, So I am always looking for the least expensive way to fix my Monte when it brakes. I come to MCF and listen to the advice of those members because they have experience with these cars. Sometimes I agree, sometimes I don't. I add there experience with my knowledge and have taken that Yellow Monte SS from neglected to Loved. A lot of that is thank to the advice of MCF members. They, We will not steer you wrong!

 

I appreciate the advices I been given but the catalytic converter theory makes much more sense. I guess I'll send y'all a video on how my car be acting when its driving.

 

before i was gonna buy the cat, i resetted the computer on my car to make sure it was cat. i restarted the car and now im getting code readings besides the p0300: p0122-tps, p0128-coolant thermostat<regulating temp., and p0404-egr.