Why is the coolant boiling out? on 2005 Chrysler Sebring

Temp gauge reads just below the 1/2 mark.
After a few miles coolant is boiling out the overflow tube.
I tried the heater and it will only blow hot after a min or two at 1500 rpm or higher. The radiator fans only kick in after the heater get to the point its blowing hot air.
At idle the fans do not run even tho water is boiling out.
I am wondering if this is a thermostat or a temp sensor.

Asked by for the 2005 Chrysler Sebring
it sounds like you have damaged the motor from overheating and blown a head gasket. you need a shop to do a leak down test to confirm and give you a bid.

i forgot to add... this happened on the trip home from having an emissions test.
emission test is a plug and play. no real connection to the failure.

Thank you so much for the help.
People like you help us idiots out so much.
I did find that the issue was the radiator cap was not tight.
It was just locked on not making a seal.
Now I feel even more like an idiot.