I have this thermal printer main board that isn't working. The separate power supply that provides 25V and 5V is fine. I have narrowed the problem down to this power MOSFET. It took a while (no thanks for the dumb minimal marking, DI), but I found the datasheet.
I believe that since I have 25V on the (red) source pins and 24.5V on the (orange) gate pin, there has to be 25ishV on the output, but there's -0.2V (blue). The SOIC8 chip that blue leads to is a DC-DC converter that creates the 5V power rail and further on 3.3V. Since neither of these voltages are present, logically nothing works. The 5V output from the power supply board is a separate 5V rail for something else. It does not power the logic.

There is nothing even remotely burned or damaged on this board. Schematics are non-existant, so I'm just following traces and trying to make sense of it. Is there an in-circuit way to definitely measure that the MOSFET has failed internally and needs to be replaced?

 

That MOSFET is an enhancement mode P-channel and thus should be off - high resistance between source and drain - when the gate voltage is nearly the source voltage as you’ve measured.

 

I just realized this and came back to edit the post.
So it looks like I'll have to trace back those transistors under it to find whatever is supposed to be turning the MOSFET on.