Code 15

My car needed a new coolant level sensor and a new engine temp sensor. After changing both of those sensors I now can see my engine temp but it still says low coolant and now i have a cel.

When I pulled the code I got a code 15 all i can find online is resolve all overheating problems which i have none and check wiring to ecm. I also noticed the the sensor in front of the engine temp sensor has nothing plugged into it I don't know if it is suppose to but there is a connector there that doesn't match the spare plug thats also up there if anyone knows which sensor thats suppose to be it would be really appreciated.

The coolant temp sensor is a 2 wire affair, not the single wire for the dash gage.

The one I changed had two wires connected to it underneath the engine it was right behind the alternator. When I changed it the dash gauge came back on and started telling me how hot my engine was. Also when I started changing it my car got the code 15 and started overheating.