Hello, I'm new to the StangNet forum, but I've been doing DoorDash for a two months or so, and my mustang started overheating. I first suspected the thermostat, so I changed that and added new coolant. But whenever I was at a stop or basically not driving down the highway the engine would slowly overheat. So I thought, well it can't be the thermostat because it's new now, so I went to my Chilton Mustang Owners manual and suspected that it might be the engine coolant temperature sensor. Finding and removing the engine coolant temperature sensor was a saga of its own. Mine was positioned between the firewall and the back of the engine. So I changed that out. It screwed in a little tight which I thought was funny, but I was pretty sure it was the right sensor for my model. I take the stang out for a test drive, but bizarrely enough its not overheating anymore. It's now under-heating. I could tell because when I turned the air conditioning to hot, cold air was coming out. I thought to myself, well I had cleared the codes in the engines computer using my obd, so maybe it needs to collect more data and it'll return to normal. So eventually after a few drives, it gets to a lukewarm air temperature. Except now I'm getting terrible fuel economy. I think when I drove my car for like 15 minutes it dropped a quarter of a tank, but then started to slow down. I didn't notice any black smoke from my exhaust and I went back to the Chilton manual and to my obd. The obd threw me a Thermostat code, and again I just changed the thermostat. So it has to be something with the ECT sensor. Maybe I screwed it in too loose or too tight or something, but why that would make my engine undercool, I don't know. Any suggestions about what might be the problem.