Noticed excessive frost buildup on evaporator coil. Checked electrical continuity of defrost coil and found it to be good. Manually indexed the defrost timer and found that it was not sequencing properly (as described in You Tube video). Ordered and replaced the defrost timer. I did not have the means to cool the defrost thermostat to check its function so I ordered and replaced it as a precaution, Unit now works properly.