Chevrolet Forum - Chevy Enthusiasts Forums - Check Engine Light Appeared Overnight?

Hi all. I've got a 2013 Chevy Sonic LT 1.8L. Sunday I got an oil change at Valvoline, drove home fine. That night I restarted my car and the check engine light came on. Drove it a short distance, the engine went rough and stuttering, check engine light started flashing. I pulled over and got towed to my dealership. Found out the coil pack needed replacing due to a misfire, $330-ish total.

So they finish repairing it yesterday. I go to pick it up this morning, start the car, the check engine light is still on. They check the car again and tell me that somehow overnight, after the codes were supposedly already cleared and without being driven, there's now a problem with the thermostat/heater, and it'll be another $350-ish to fix that.

The tech said it won't hurt the car to drive it around with this thermostat problem. Is this true? Also, how could the car spontaneously generate a problem overnight? Did they break something and don't want to admit it?

I appreciate any advice, thanks!