If your 3M EC glue gun will not heat it could be a few different problems to check and see if the heater is what needs to be replaced.  Turn the EC glue gun power switch on and check the electrical continuity of the heater/TCO/switch assembly.  Use an ohmeter and measure across the terminals numbered 3-4 on the top terminal block labeled TB2. The measured value should be between 40-50 ohms.  If the measured value is not in that range then replace the heating/TCO/switch assembly using the 3M Heater Kit part number 62-9680-9731-1.