Hi, at the plug on the right hand side, J1, there are 5 wires. The green wire is earth. The blue and black wires are across the heater inline with a thermal overheat device, it should read around 7ohms. The purple and white wires are across the temperature sensor and should read 10Kohms @ 25C. Opening/closing the lid may caue the wire to make/break.
The wiring tends to break inside its insulation, i.e inside the coloured wires. You will need to remove the blue protective sleeve and then find which wire has broken. You may be able to do this by gently pulling the wires, the insulation will stretch. Most probably the break will be around where the blue sleeve goes through the casing.
I have found several of these units do this, i tend to either remove the pins from the plug (difficult) or
Carefully remove the blue sleeve, cut it off and throw it away. Find the break, resolder the wire and heatshrink it. Cut the plug off leaving enough length to solder the wires back together after.