low coolant light keeps coming on but does not stay on very long, losing coolant but don't know where. could this be from a temp sensor?
If it is not leaking out of the car on the ground, then its going somewhere, could be a blown head gasket...Are you sure you have a 50/50 mix of coolant and water. If more water then coolant then the water can be boiling out...Is the cap working? http://www.gobluedevil.com/blog/what-are-the-symptoms-of-a-blown-head-gasket/
I do have to agree with joemom if not leaking on ground has to be going someplace. Had a 2001 mercury sable doing the same thing as yours could not figure out where it was going and the vehicle was running great no smoking or anything removed top radiator hose and added Alumni Seal Stop Leak and that took care of my problem. No promises that it is going to work for you but it is worth a try.
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