Car overheats when in heavy traffic

i have a 98 saturn wagon sw2 (only 121K miles), it overheats when in heavy traffic or stopped for long periods of time. The fan does not seem to be working but where I do not know where or what to check and also it has a small leak in the radiator resevior tank under the bottom. We replaced to o-ring but it is still leaking so i am not sure if i am suppose to replace with a thicker o-ring?

Find the wires that go to the fan motor and run test leads directly to the battery. If the fan doesn't work, the fan motor is bad.If it does work, you have other problems. Probably not the coolant sensor because if the PCM detects a problem with that, it will switch on the fan as a fail safe. The fan relay is your next thing to check. Should be able to swap it with another relay in the box that you know works.

The cooling system on a Saturn does not have an overflow/reservoir tank. The plastic tank you are referring to is a pressurized tank that is an integral part of the cooling system. Overflow tanks are basically just a catch-can for the system when the coolant expands due to heat.

Your tank probably has a crack in it. Finding a replacement at a wrecking yard should be fairly easy.