One is on the downpipe (right below the exhaust manifold) and the other is right after the Catalytic Converter, and they are expensive
The o2 sensors are really not that expensive. The one that is located by the header is only $20.00 at either advance auto, or napa. The other one by the catalytic converter is only $60 to $70. If you think that is expensive then your not looking in the right stores. That is cheap compared to other parts that is normally replaced after so long.
It only has one. It is located in the exaust manifold in front of the engine. Four cylinder that is. If by chance you have a V6, then it would have two, both located in the exhaust maifolds, on the front, and back of the engine.
timing off
Never as the 1998 Chevrolet Cavalier does not have a belt, it has a chain that should last the life of the engine.
Vacuum leak
The right person to ask would be either someone who owns a 1988 Chevrolet Cavalier or a mechanic at a Chevrolet dealership. This question is too complicated to answer in a simple manner.
Around 90-100 dollars
Would have been built for R12, can be converted to R134a.
it probably doesnt like you amd wanted to drive off
Any high quality green type coolant would be acceptable for Chevrolet Cavalier. It should be mixed 50 percent with distilled water.
low fluid would cause this or the pump could be going out
Normal cause of a belt smoking is that one of the driven pulleys has siezed.
maybe a bad cam orn lobs on cams can cause that