Engine coolant temp light kicks on after I've been driving. But gage falls to cold. 2010 ford focus I just replaced thermostat

Engine coolant temp light kicks on after I've been driving. But gage falls to cold. 2010 ford focus I just replaced thermostat - 1

Check to see if it is connected to the thermostat housing. You may have left it off.

You may have an air pocket in the cooling system. Remove the radiator cap (cool engine) and start the engine and let it run for 10-15 minutes until it's up to operating temperature. The look to see if the coolant is circulating.

Did you get all the air bled out of the cooling system?

Blown head gasket.

You may have an air pocket in the cooling system. Remove the radiator cap (cool engine) and start the engine and let it run for 10-15 minutes until it's up to operating temperature. The look to see if the coolant is circulating.