I had replaced the defrost thermo a month ago. After a few weeks the coils froze up. Removed the glass coil and took into for test, they said it was fine. I removed the defrost thermo and tested it cold (analog ohm meter) showed 0, let it sit all night to warm up, tested warm showed 0. Just received my replacement defrost thermo in the mail today and before installing I tested and warm is showing 0, which from looking here Refrigerator Repair Guide: How To Test the Defrost Thermostat - ACME HOW TO.com
would mean this new replacement defrost thermo is bad as well.
Using an analog meter I have it set to R x1K, touching the probes together to set zero.