They say every 30k miles you should do a coolant flush but is it necessary and what will happen if I don't do it?
I believe our maintenance interval for coolant is at 50k. Might wanna call your dealer just to be sure. Coolant flushing helps keep the water pump lubed up. Over time tends to "brown" cuz of rust so you might not want anything like that blocking your coolant passages. So yes, I find it necessary.