Engine temperature sensor.
But when I had this occur on my 92 Merc Sable it was due to a failed water pump. No circulation of the coolant means that the car will overheat.
Having it chemical tested will tell a lot more about the circulation of your coolant than just head gasket issues. Might be air in the system, no flow through the water pump, etc.
Thursday, February 2nd, 2006 AT 7:28 AM