That's an N-channel insulated gate bipolar transistor, IGBT, rated at 1200V and 81Amps. Infineon Tech.
https://www.infineon.com/dgdl/auirg4ph5 ... ba31da1519
How many are inside the unit-- 4 or 6 maybe? or do you have a picture?
With it open you may be able to measure the PTC heater resistance with a voltmeter.
sounds like you are making good progress.
 i noticed that IGBT is missing an internal body diode, wonder if there are diodes on the board to perform the freewheeling function when if is switched off with current flowing? That could easily blow such a device especially if there is inductance in the load.