After Hours Appliance LLC

The defost system usually has 3 parts to it:  heating element, bi-metal (thermostat), and  timer (circuit board or mechanical timer).

I usually check the element and the bi-metal, first. If the unit uses a circuit board to control the defrost function, this usually requires a certain routine to initiate the cycle.

IF you can confirm that 2 of the 3 are good, than that leaves only one that it can be.

Bi-metal should  close at 20-45 degrees F, also check for this to happen.

Heating element should show roughly 20 ohms of resistance.

Timer if it's mechanical should have a little hum to it and can be manually advanced to the defrost cycle. The cycle should last around 30 minutes. When in this mode check for 110 volts across the heating element.

IF all the above checks good and you need our help, just give us a call.