I would start at the thermostat. Problems with the radiator and head gaskets are usually presented as visual leaks. But clogged passages in the radiator would also cause overheating issues, so have the cooling system flushed.
Take off the hoses!
I checked my owner's manual - it listed radiator capacity at 6L.
driver side of engine where in/out radiator hoses connect.
It could be the radiator is clogged. The thermostat could be bad (prevents the coolant from circulating from radiator to engine block)
Front of the radiator, just under the opening of the air intake.
It's the power steering oil cooler.
Folllow your upper radiator hose to the engine to locate the thermostat housing
I would recomend getting the Haynes manual
The air inlet temperature sensor is found inside the intake of a 2005 Hyundai Elantra. More specifically it is located near the air box where the filter is.
I tried to remove my Hyundai Elantra 2002, im having a hard time pulling out the hoses. There is one screw that you have to remove on right hand side of the radiator, just beside those freon connection. I used a very small 10mm rachet.
Follow the upper radiator hose to the engine and it will lead you right to the housing of the thermostat
Bad temperature sensor unit? Cooling fan not working? Water pump not circulating coolant? Radiator plugged or restricted? System airbound? Thermostat not opening?