Second time this has happened. First time, about 3 weeks ago, I replaced the defrost timer. Before that, the heat cycle would not engage. There was ice, not frost, built up on the coils. All the ports to the lower cabinet were blocked with ice. The new defrost timer DOES engage the heater with a sizzling noise. I asked a local repair guy what else it could be. He told me to apply hot water with a turkey baster and make sure the drain line is not clogged -- which it was. I gathered from this that the clog would cause a recurrence of the same problem. Did this take care of the problem or should I suspect a defrost heater? Or even a thermostat? Trying to be proactive so I have the parts ready. Thanks for your help!

Yes it was frozen/clogged. I made sure the hot water was getting down to the drain pan. I did not unclog the drain the first time the unit iced up so I am hoping that this will be the end of it?

I am also going to replace the bi-metal thermostat. This is a rental property and the tenant has already twice had to throw spoiled food away...an ounce of prevention...