Yes, most definitely.
A malfunctioning EGR valve can definitely lower fuel economy.
Yes, the EGR valve, or port EGR
Blocking off an EGR valve can make the car run rough. The fuel mileage can suffer as well, since the car is designed to have the EGR valve operable.
Worn piston rings and or clogged EGR system and or failed EGR valve
Would this cause the car not to start If the EGR VALVE is bad
A bad EGR valve in a 2000 Saturn can have several symptoms. The EGR valve can cause misfires, the valve can cause vacuum leak style problems. There could also be an engine knock.
Yes
Stumbles, dies off, low power, poor fuel economy, check engine light
No, the catalytic converter is downstream from the EGR valve so it has no effect on it.
Even a good EGR valve may cause an engine to stall in hot weather.
A faulty or clogged egr valve can cause an engine to run rough. If stuck or clogged the valve can be cleaned after removal with carburetor cleaner.