Hey all. First a little background. I have a '15 Mustang GT with a Roush SC and the Roush tune. The car has never had any work done nor has it had any overheating issues.

The other day my wife pulled into the driveway and shut the car off. A few minutes later we went outside and tried to start the car and it would crank, but not start. The "Coolant temp high" prompt came onto the screen and the CEL came on. I checked the coolant degas bottle and it was indeed low, but I did not think low enough to cause any issues. I topped it off to a little over "high" and started the car. The problem went away and we drove it to my dad's house, which on the way there did the same thing and then went into limp mode.

Right before the "Coolant temp high" prompt comes on and throws the code, all the temps look perfect. The analog needle is on the low side of the halfway point, the "Engine head temp" readout in the gauge menu reads 194 which is where it usually hangs out, and the "oil temp" readout on the gauge menu reads "normal". Then as soon as it throws the code the needle goes all the way down to "cold" and the "engine oil temp" just shows dashes. If I use my scanner tool to reset the code all the gauges and readouts go to normal then once it is done resetting it will throw the code and do it again. All the while, the "engine head temp" stays constant at 194. I also noticed that during the CEL reset using my tool, the "engine head temp" will go all the way down to like -30F, and then when the tool is completely done resetting it will climb to 194 then throw the code. At first it was only throwing P1289. After a few times of trying to figure out why it was doing this, it started throwing P1299.

So far all I have done is change out the thermostat and gave it ample time to burp any air out of the system. I have no idea why this is doing this. We did not touch anything nor was any work done that could have caused a short or the plug to the sensors to be hit. The only thing that might have prompted this was the coolant being a little low.

Any help would be greatly appreciated, I am at a loss.

 

Solution: replaced cylinder head temperature sensor. Fixed the issue. I have no idea what caused the sensor to fail.

 

Glad it was a simple fix.

 

Glad you figured it out. Did you find it on your own, another forum, or take it to a mechanic? Just wondering. Sounds like a strange problem.

 

I have a 2015 GT premium with 15k miles and the exact same issue. Please let me know where the sensor is located so I can replace it as well

 

It's been awhile, and I have since parted ways with my stang (which was one of the hardest decisions I had to make. I still cry about it). But if I remember correctly it is on the backside of the passenger side head. Very close to where the head mates with the engine block. I hope this helps.