How do you replace freon in AC System for 2000 SL1 Saturn?

First, never add refrigerant if the system isn't truly low. You'll need BOTH gauges (high and low) to know where the system is. A worn compressor will fail to build sufficient pressure to keep your vehicle cool.

Never add refrigerant if you don't know the condition of the system. It may not have needed refrigerant and adding too much could have shut the system down. The 2000 Saturns have a shutoff switch that prevents the compressor from operating if the pressure is too high.

There is no need to purge the system unless it has been completely evacuated and air was allowed in.

Sometimes an AC system will not work when the vehicle is not moving, especially if the radiator fan is not functioning properly or if either the radiator or condenser coil are blocked, preventing air from blowing across.

For future reference, your 2000 Saturn does no use "Freon", it uses 134-A refrigerant.