hey man i went through all this. i have a 94 we tested everything from the switch, hoses, coolent temp. censor, cylonoid, and vacum, everything checked out. but i found out for my year you have to have the tires spin freely for the egr to open. still the egr code poped up, what happens alot with the cars is the passage ways to the egr get block, get carbond up and i am quoting a Suzuki repare man on that, and what you can do is take the EGR off clean it with a wire brush then clear all the ports and your prob, should be solved, your going to wana replace the gasket though, i havent done mine yet because of time, but on your question of what does the EGR do? it takes carbondioxide from the motor and mixes it in with the air fule mixture, this is how u get good gas mileage, with my EGR blocked im getting about 25 but thats with a cold air intake, and full 2 and 1/4 exhaust so i should be getting alot better. any more ?'s just ask