The tune up should be part of the routine annual service - maybe the car hasn't been serviced or maybe it hasn't been serviced very well.
I wouldn't describe a sputter as a misfire but undoubtedly some folk would - nothing is more confusing or misleading than the different terminology folk use to describe the same thing.
If sputter is an accurate description it suggests momentary fuel starvation.
I suggest you initially don't worry about what might be wrong with it. The car is probably ready for a grand birthday where just about everything is looked at, studied, cleaned and adjusted or replaced and generally poked and prodded by learned men and in the process the current problem might be cured.
Considering your post it is the best starting point I can suggest.
SOURCE: 02 monte carlo, radom misfires, EGR circuit failure
Does it misfire under load like when you press on the gas peddle after slowing down then accelerate or at idle. When you say spark plug did you mean all the spark plugs?
SOURCE: 97 Chevy Monte Carlo won't start, security light is on
disconnect battery for 24 hrs so the computer will reset and if that dont work take it to chevy dealer and have them reset it manually
SOURCE: multiple misfire 95 monte carlo 3.1 liter
check codes first. But sounds like crank position sensor mine does it all the time
SOURCE: power loss
Can't answer your last question but catalytic convertors that are just not working anymore won't cause any loss of power.
However, if they have shattered inside (they are kind of a ceramic honeycomb) they can dump junk into the exhaust system that can eventually clog your muffler or resonator.
Checking whether this has happened isn't difficult; after the warm up of the engine, either you or a helper needs to listen to the exhaust note.
If firing of the cylinders is still distinct, the system isn't clogged with shrapnel; if it seems to hiss at higher RPMs, it is likely clogged.
This effect is easy to miss; I've had two failures and when cold, the engine would pull fine because the chunks of ceramic would fall to the bottom of the muffler and glue together somewhat.
Once hot and agitated, they would clog the muffler and cause a severe loss of power but the idle would be OK.
A hill that I would normally pull at 70 MPH, I couldn't top at all; had to sit and wait for things to cool down before proceeding.
It also seems some engines appreciate a bit of back-pressure in the exhaust system because the car gained in mileage and pulling power for several thousand miles before the clog became critical causing the mileage to fall sharply along with the power.
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