I have a circuit that uses a TRIAC that controls when 120VAC passes to the output. Never really had any problems until recently when I noticed that I had continuity through the TRIAC without any power going to the gate. I have 100 other boards that don't have this problem. So my main question is, is it normal for a TRIAC to fail closed and if so, how does this happen? Manufacturing error perhaps? My knowledge on TRIACs is pretty limited. The only reason I am so concerned is because I use this TRIAC to control power to a heating element. Without any control on the gate the temperature just skyrockets.
Oh, the TRIAC I'm using is: MAC4DLM.
Schematic: https://ibb.co/f4QPAk Thank you!