The Thermistor Repair Kit might be needed if your 3M Scotch-Weld EC glue gun will not heat. Check the continuity of the Thermistor disconnect the Thermistor lade from terminal number 1 on the upper terminal block (TB2).  Use an ohmeter and measure between the disconnected Thermistor lead and terminal number 2 of terminal block TB2.  The measured value should be between .65 and 1.25 Mega-ohms, if it’s not in that range replace the defective Thermistor. The 3M 62-9680-9733-7 EC Thermistor Repair Kit includes a trigger pin, Thermistor assembly handle screws. end clip, e-ring, heat block inserts (1 set) and a spring.