coolant temperature sensor

If the reservoir was empty, you will likely have to keep checking it as air burps out of the system. I would guess you have a leak somewhere, but the code may have been from low coolant. I've not had any coolant issues on mmy Smart so I can't direct you to where the thermostat is, but I have had these issues with other cars and it should be pretty much the same. Problems I've encountered in order of frequency are thermostat stuck (open or closed), leaky/ruptured hoses, radiator fan died, failed water pump.

Have you cleared the code and had it return?
Is the coolant level stable or has it dropped again?
Does the fan behind the radiator come on?

Thanks for the info. We have not cleared the code. Husband is thinking he needs to replace the temperature sensor. The coolant level is stable. I don't know about the fan. Where is the radiator?

The radiator is not easily accessible without taking the front bumper cover off. There's nothing much to do about the radiator. Fill-up and monitoring coolant level are through the coolant reservoir, anyway.

The code was likely because you're engine was overheating. Hopefully it didn't warp the head. You have to burp the system to remove trapped air. There's a coolant hose in the transmission side that you need to remove and wait until coolant starts pouring out and then hook it back up.
Sadly smart don't have a coolant temperature gauge and you often get a check engine light when it's too late

Thanks for the help. Coolant level is holding and the check engine light went out. Once before it came on because the gas cap was lose, and didn't go out until the next time I put gas in it. I only want to drive a smart car forever, and will keep a closer check on the fluid levels.