Pre-zackly!insta wrote:the MOSFET itself sticking on
Although you'd prefer transistors to fail open, the usual failure welds the source to the drain as a dead short: whatever's connected to the transistor will be stuck on forevermore.
If that's a heater, it runs at 100% duty cycle until something else fails / burns through / gives out, because the firmware has completely lost control.
The other common failure has the firmware locking up with the transistor turned on. After the firmware vanishes into its own navel, it doesn't pay attention to anything else and won't turn the transistor off no matter what the sensors report. From the outside, both failures look pretty much the same, with full-on heating until something else burns out. With a firmware lockup, however, the controller survives: reboot that sucker, it looks around, goes "Who? Me?", then starts running like nothing happened.