The most common cause is the coolant level is too low to circulate through the heater core in the dash. That was the warning that might save the engine. Now it's low enough that it's not circulating through the radiator. The overheating is a common cause of a warped cylinder head which will leak coolant, and a leaking cylinder head gasket is a common cause of coolant loss that leads to overheating. Your mechanic can do a chemical test at the radiator cap to test for a leaking head gasket. If it needs to be replaced, once the head is off he will check it for warpage. If it is warped it will need to be machined or straightened, depending on which engine it is.

Answered by caradiodoc (expert)
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