Diagnosing sub-zero refrigerator defrost issues the proper way.

I was asked how to accurately diagnose a defrost issue on a subzero refrigerator.
 The answer to that is to use common sense and technical skill to determine the part most likely at fault in the defrost system.  Your defrost system is primarily controlled by your control board, which sends the power to your heaters.  In between the heaters, there is a switch called your terminator switch or “the thermostat.”  this is a heat sensitive switch, and when the freezer is at temperature, it should be closed and allow power to the heaters.
 to diagnose this problem in an older subzero unit the power for the heaters with originate at the defrost timer instead of the control board.
 I hope this helps and shows use some common sense insight into how to diagnose these issues.  sometimes it can be quite tricky but using you volt meter and common sense will eventually fix your problem.