Radiator losing coolant. No water in the oil. No apparent leaks. Car wont start sometimes after it has been ran awhile. Changed the thermostat. Plugs & wires as well. What is wrong?

Sounds like head or intake gaskets. Have it pressure tested at a REPUTABLE mechanic.

Blown head gasket. Usually. Thousands of dollars. Sorry.

It does sound like a bad head gasket, for about 40 bucks you can get a test kit for the radiator. it involves pulling some air from the radiator through a test fluid that will detect combustion gasses in the cooling system. Not that hard to do, just follow the instructions on the kit. there are some good youtube vids on that too.

I have owned several Hyundai's from Santa Fe to Elantra(4). All of them would start experiencing coolant loss with no trace of where it would go. I consulted my Hyundai Dealer Shop Manager. Very bright individual always puts me on the path of issues that Hyundai has. This issue involves the "spring loaded" Hose clamps for the upper hose,bypass hose and lower hose. They get weak with time and the hose hardens with heat and age. Suggest doing the following. Thourughly fluff the system. First remove all hoses from engine and radiator. Obtain new hoses. New worm screw clamps and replace the thermostat. Fill the system with proper mixture of coolant. Allow it to run until hot then re- torque all of the hose clamps. Make sure to clean out the overflow bottle too and fill it to the proper line. This should cure the disappearing fluid issues. I have hundreds of thousands of miles on all my Hyundai cars. Never had a head gasket issue under normal driving conditions and care.

2005 Honda Elantra is leaking antifreeze the thermostat has been replaced.. and the hoses have been replaced .. what else could it be

Pinhole radiator leak?

I turned out to be a freeze plug in front of the engine. It had a small leak but you couldn't see it because the engine mount was in the way and could not see it. Replaced the plug and no leaks. Done!

Before buying a car: you can test to see if the head gasket is ok by taking it to a radiator shop and have them test for exhaust gases in the coolant. Done in 5 minutes....

Water from overflow bottle not returning to radiator after car has cooled