basic electronics question

R53 is likely sensor part of a current limit for Q6. The clip of schematic is not enough for me to get full understanding. Typically in a single low side switch, a source resistor is viewed by a base to emmiter if a small bjt, and the collector pulls fet gate low, for off when Vbe is about 0.65V. Q2 and Q4 may either provide bias for Q6, or part of current limit. C33 is large and may slow down kick in of limit. By pushing the amp to limit, it should be possible to view R53 voltage, and Q6 gate to see action of current limit. Assuming it works....

D16,17 are back to back 15V zeners, they should limit gate voltage to + - 15.7V. Q6 gate limit is + - 20V. Q6 is hefty, but could fail if open in D16,17, or long term overload. I would certainly test Q2, Q4. They are small, and if Q6 shorted, they might fail too, or failed Q6 without current limit.