I sympathize but there's a reason shops charge that and emissions techs need to be licensed. The stuff can be tricky to do and requires special gear and knowledge. My point was that if you'd had even the basics down you should've gone right to the O2 sensor after seeing the numbers. I hate to see someone flailing around when it's gonna end up causing frustration, time, and money. You're learning something true, but learning works best when you study the academics first and emission stuff requires that if you want any chance at all of being successful.
The other problem is the IM 240 test (what your state uses) is a very tough one. Even if you had a grip on this, without the gear to verify your repairs you're still at the mercy of the state. Trying to solve these issues without a background and proper gear isn't easy.
If the sensor is original go ahead and change it. Change the plugs and air filter. Change the oil. Forget putting anything in the tank. In fact, other than making sure the car is good and hot (cat lit off), most of the techniques out there for passing emissions are a total crock. Even the stuff they sell to put in your tank is nothing more than poly-amine fuel system cleaner. The only proven way to pass a test is to make sure the engine is mechanically healthy and running at stoichiometric feeding a good converter. The car passed when it was new in just this configuration.
You can learn this stuff lots of places but here's some basics:
The cat can only work when it's fed a very narrow range and composition of gases. That's the job of the O2 sensor. The sensor's sole reason for life is to serve the cat converter. Sure, it makes the engine run "right" but all that does is get you gases in the right amounts the converter needs to operate. This is called the stoichiometric mixture. It's the theoretical mixture where all the fuel and all the air is consumed and nothing should be left over but CO2 and water.
A stoich mixture is 14.7 parts air to one part fuel and stoich is what your O2 sensor is calibrated to. If the cat sees anything much richer or leaner than stoich it's efficiency, it's ability to clean up CO, HC, and NOx, rapidly drops off. Why? Because it can only be so big and thus can handle only so much (even when new). Don't buy "cheap" replacement converters unless you only want to pass a test. Cheap means small because of the precious metals used in cats. Small means they'll handle even less pollutants than a good one. With cats you truly get what you pay for.
Anyway, in the old days stoich was "good enough" until the eco-nazis in California decided emissions had to be cleaner. Thus the cat was born. It cleans up the little bit of mess left over but the key thing to remember is *it must be fed a stoich exhaust stream to work*. You can beat down emissions from a non-stoich mixture with a new cat but the cat won't last long.
So it's obvious the key to good tailpipe numbers is first and foremost making sure the engine is running in closed loop at stoich. This is why emission techs and mechanics are trained to look at the exhaust stream *before* the cat when diagnosing emissions issues, because the cat can mask engine problems unless you're using a 4 or 5 gas analyzer. For emissions test themselves all the state cares about is what you're dumping into the air. It should be clear by now that just because you pass an emissions test doesn't mean your engine is running 100% right. Close probably, but your cat may be taking a beating just the same.
Anyway, if the engine is running in closed loop at stoich the CO will be around 1%, CO2 will be around 14-15% (the higher the better), HC will be around 50-200 ppm and Nox at around 1500-2000 ppm. These are "engine out" emissions, the cat will further clean this up to even less. Btw, the cat has two different sections inside, one for cleaning up the CO and HC and another for cleaning up NOx but we'll skip how it works beyond that.
When the O2 sensor is constantly switching above and below stoich the engine is in closed loop ie; it's responding to changes made by the ecu to keep it right around stoich. Lean, rich, lean, rich. You can see this by looking at the O2 sensor signal. It should be switching high and low but averaging about 450 millivolts. That's the mid range of the sensor where stoich occurs. Any average above or below that means that, although the engine is still in closed loop, it's been shifted lean or rich of the 450 mv stoich midpoint. If the sensor signal is "stuck" high at around 800 mv the car is open loop and running rich, stuck low at around 200 mv and the car open loop lean. Either of these conditions aren't stoich and the cat will not like being fed this rich or lean mixture. Converter efficiency falls and emissions go up.
Why the switching on either side of rich and lean? Why can't we just run the engine at dead nuts stoich? Well, the cat actually needs to have this switching. Every time the mixture goes lean there is extra O2 in the stream and every time it goes rich there is extra CO. (O2 is a product of lean mixtures and CO is a product of rich mixtures). I'll spare you the details but suffice to say the cat needs these extra bits of O2 and CO to work. So the cat needs a mixture swinging slightly above and below stoichiometric to be happy. Feed it that and you'll be golden on emissions. It's as simple as that, in theory at least.
In practice lots of things can cause the mixture to drift from stoich. Remember, the O2 sensor only measures one gas: oxygen. It has no idea what the other gases are but if the mixture is stoich the 02 will always be around .5%. There is a definite relationship and interaction between each as in the exhaust stream so this is how we can use an O2 sensor to control to stoich.
That's the easy part. Your job is to find out why the mixture isn't stoich or in other words find out why the mixture has more or less O2 than around .5%. Not an easy thing to do without the right gear and knowing what causes each gas to change. As a rule this is what's involved:
CO means a rich mixture. O2 means a lean mixture. CO2 is an indicator of combustion efficiency, the higher it is the better and more complete combustion was. It peaks at around 15% and an engine at stoich will always be close to that. HC is unburned fuel. Period. It's gasoline that wasn't combusted. Usually caused by an ignition problem (misfire) but can also be from things like valve deposits, oil diluted with fuel (it's why you change your oil before a test), a saturated EVAP system. ect. Anything that makes gasoline go out the tailpipe unburned will cause high HC..
NOx is due to high combustion temps over 2000 degrees. It's only at this temp and above nitrogen in the air (which normally passes unchanged through the engine) combines with oxygen to form NOx. Causes are a faulty EGR system (reducing NOx is it's sole purpose), bad cooling system (high engine temps), bad fans, incorrect timing, anything that causes high temps in the engine or cylinders. NOx is a tricky bugger because it's maximum occurs very close to stoich. Let the mixture drift just a bit lean and NOx shoots up. The mixture must be controlled right at stoich so once again it's the place to start.
Those are the basics (trust me, there's lots more to know) but armed with that you have a chance. My point is any time the numbers are way off the first place to start is with the engine in closed loop and the O2 sensor is at the heart of it. Good ignition, fresh oil, clean air filter, and minimal carbon deposits are also a must but feed the cat stoich and if it's healthy it'll do it's job.
Whew. Good luck...
