Being an older fridge there isn't much to these GE side-by-sides as far as electronics. To be honest, I had no idea how the defrost circuit worked in a fridge, but after a quick web search learned of the heater circuit. I pulled the freezer back panel (four screws) and checked continuity of the heaters. I had none at the plug. Removed the heaters for visual inspection and individual continuity checks. Four more screws and the two heaters were on the kitchen table. One was clear, the other was dark and had a green color to the glass. That one had no continuity. The way the circuit works is in series, so if one goes bad, the other won't get power. Ordered the factory part from AppliancePartsPros (probably my third or fourth parts order from them over the years) and had it in two days. Installed it and by the next morning the fridge was cycling fine and nice and cold. Easy repair and our old GE can hang in for another year until we reno our kitchen and get all new appliances then!