My car over heated so I went and got a diagnostic
test and told me to change the thermostat.. So is
the thermostat the Most common reason for over
The Thermostat is usually the 2nd item to check. First make sure the cooling system is full and the overflow take is at the correct level. Overheating can be caused by several issues. Low on coolant, leaks (internal or external), clogged radiator, bad thermostat, bad radiator cap, etc.
Probably the most common reason for overheating is air trapped in the system after its been drained and refilled to fix some other problem, but everything Gary said above is true as well.