Q: Where is the cylinder head over protection sensor located in 2015 ecoboost mustang

Hello. From what you describe it would appear that the sensor you are referring to would be the coolant temperature sensor, also known as the engine temperature sensor. As it’s name implies, the coolant temperature sensor monitors the coolant temperature and sends that signal to the computer so that the appropriate engine management adjustments can be made. The sensor is located on the right side of the engine, installed in a water outlet housing just above the turbocharger. If the code is still present, even though the engine is no longer overheating then it may be possible that the code just needs to be cleared, either by driving the vehicle a specific number of cycles, or with a scan tool. If you do not have access to a scan tool, I would recommend having a professional technician, such as one from YourMechanic, come to your location to (scan and clear any remaining trouble codes)[https://www.yourmechanic.com/services/check-engine-light-is-on-inspection].