Why does my heating fail

I have an Ender 3 pro with the 300XL Ender extender kit installed. I was still using my original 235 x 235 heating pad under a 300 x 300 aluminum bed. On top of the Aluminum was a 12 x 12 mirror. This setup worked fine as long as limited to 235 x 235 print area. I was busy printing small domes for my daughter and didnt want to take time to change firmware settings. Short while after finishing domes , I purchased a 120 volt 750 watt 12 x 12 silicon heating pad and silver insulating pad to help hold heat in. I didnt install right away. I started hearing a slight crackling sound. Thinking the 235 x 235 heater was on its way out I changed it. Another reason I changed it was I would get this message "HEATING FAILED PRINTER HALTED PLEASE RESET". So with a SSR I replaced the bed hoping was the error cause. NOT SO, I still got the same failed heating messageNow I needed to confirm what was failing. Hot End or bed? I heated up Bed inpendently and that was not the issue. The 120 V heated up a lot faster and with no error messages multiple times. The hot end was different. Thats where the message would pop up. If I heat the nozzle up to 200 and get that message. I did a test, I heated the nozzle to 100*, then 125, then 150.....heated nozzle ok Now try 175, at 164* the heating failed on the nozzle WYF Now I wonder if its the little heater inside that aluminum block is at fault. I checked the connections when I replaced the thermistor inside the mainboard box. Still the 8 bit board in also. one of these days Ill install the SKR 1.4 turbo and all its goodies. BUT need to print a new housing before due to placement now of new extruded in the way on EX kitAny Ideas? Am I on the right track about heater barrel going south?