Sync and Drive throw codes but no check engine light and diagnostic tool does not pick it up

I came here looking for an answer for this same issue. A couple of years ago Sync showed me a trouble code. I took the car into the dealer, they said their machine showed no active or stored trouble codes. … Earlier this week Sync sent me a trouble code that said “Coolant Thermostat Malfunction” “Coolant Thermostat failed, replace” but there were no warning lights on the dash and the gages looked fine. … I took it to the dealer and was told that their machine didn’t see any active or stored trouble codes but they replaced the thermostat anyway because it’s covered under the warranty and would rather be safe than sorry. (Guessing since I showed them that code had popped up and they didn’t replace it then the engine overheated and had to be replaced they’d be liable?) On my way home from the dealer, the same code popped up. Still no warning lights on the dash, gages still look fine. … AND I missed a wedding I was planning to drive across country for due to the car being in the shop when it might not have even needed to be.