I suggest you measure the voltage at the gate and at the drain of TR3. The part number for TR3 is BSH105. With the heater commanded off, the gate should be at +3.3V and the drain should be at 0V.
If that's how they are, then TR4 (the main E0 mosfet) is shorted
if TR3 gate is at 0V then either TR3 or the microcontroller is faulty, or the firmware is turning the heater pin on. Check that you don't have a M307 H1 A-1 C-1 D-1 command in config.g to free up the pin for another purpose.
if TR3 gate is at +3.3V but TR3 drain is at 5V instead of 0V, then TR3 is faulty.