I have a 2009 Justa Cooper with 37K miles. About a week ago I began to notice I had lost my fast idle on start-up. Some times it was ok, some times it would run for a few seconds and stall. I could not find anything specific on this forum, so when it got worse this past Sunday and Monday, I decided to leave the car with my mechanic overnight, so he could have first crack at it in the morning. He connected to the computer, and found the coolant temperature was 143 degrees after sitting all night. This indicated a bad ECT Sensor (Electronic Coolant Temperature). He replaced it, the coolant temperature went to 80 degrees, and the engine went to 1200 RPM on start-up. $140 to repair.

I hope this helps another owner with a similar problem.

 

Thanks for posting.