Have a three year old Kitchenaid Undercounter Fridge KUDF204ESB00 sn: EK81003346

Unit cools but ice starts building up after a few days.

I have found a Whirlpool manual for an older version of this unit. With it, I have confirmed the following:
1. Refrigeration cycle: is working as the manual says it should. The following tests out as good during this procedures: Wiring harnesses, compressor, Thermistors, fans, controls, etc.
2. Defrost cycle: The controls have changed for this unit so I do not have the sequence to enable the Defrost test cycle. However, I have tested the following as good: Wiring harness, Fans, & thermal fuse. There is no separate timer. The fridge is on a 10 hour cycle. I believe the timer function is on the main circuit board. When I started troubleshooting, I decided to role the dice and replace the main circuit board. So, I'm pretty sure it is fine.

The only thing I can't really test is the heater loop but I have ohmed it out. It shows some, but not complete continuity. (showing 103u.) Based on what I've read, this should show complete or nearly complete continuity. Might this be my problem? On some parts pages, this part is listed as "Bi-Metal." What ever they mean by that...

Also, I would very appreciate it, if anyone knows the button sequence for activating the system Defrost Cycle and if you can answer my question above regarding, what I'll call the Heater Loop.

Thanks,