Heater Core CEL Problem?!?

On my first test drive for my swap i blew my heater core, had coolant/water all over the passenger side floor board. (luckily didnt fry the ecu) Wasnt a big deal at the time, but now that its getting cold i need heat! So i got a new heater core, had the dash pulled in no time got it in there and all the lines ran. Started it up and burped the system. It never really got "hot" just warm like cold bath water. Its better then nothing, but a brand new heater core i want it to be a sauna in there. So i dont know if there's just still an air pocket or something in the system or what? But now i also started to throw a "check engine" light! Even with my swap the ecu is chipped and ive never thrown a C/E so im not sure if ive messed something up. I dont see how but always a possibility....any thoughts on what i could have done? Or whats going on? Car runs fine still just worries me that im throwing a light now.
Comments
What is the code? There ain't one for heater core...
yeah it could be the thermostat, i dont even have my fans wired to run at all right now. im gonna put them on a switch, its just been so cold out i dont need them.