Subzero 550 coils

WOW. I DISCOVERED THIS WELL. I HAD PROBLEMS FOR YEARS WITH THE DRAIN SYSTEM FREEZING UP. I DID THIS ALL WITHOUT PULLING THE FRIDGE AWAY FROM THE WALL. I DISCONNECTED THE DOWNSPOUT AND SNAKED/WRIGGLED A FLEXIBLE WIRE DOWN THE DRAIN AS FAR AS POSSIBLE. I THEN REPLACED THE DRAIN BRINGING THE WIRE UP THROUGH THE DRAIN AND BEHIND THE RUBBER SEAL SO IT IS IN THE UPPER PART OF THE FRIDGE(ABOVE THE CRISPER) AND TAPED IT TO THE BACK WALL. THERE IS ENOUGH HEAT CONDUCTION TO PREVENT FREEZING. IT'S BEEN OVER A YEAR AND NO FREEZE UP.

I had same problem very recently.  The problem with both the refrigerator and freezer on the 550 is that the drain line that takes condensation and water from the defrost cycle to the drain pan freezes up. Sub Zero parts suppliers will sell you a fix-it kit which I used for the refrigerator.  The kit is simply a heater wire that you plug into one of the existing circuits and runs down the length of the drain pipe to keep it from freezing up.  When my freezer started doing the same thing I ordered the kit for the freezer but the wire was too fat for the drain pipe so a Sub Zero rep gave me a better and more elegant solution.  You need a length of bare 12 guage copper wire, about 24 inches.  Run it through the drain pipe with about 6 inches exposed at each end.  On the underside of the fridge, wrap the end of the copper wire a few times around the black heater coil that you will see running back and forth.  At the upper end, wrap the copper wire around the black heater coil that defrosts the freezer coil.  The heat generated by the two coil will simply conduct heat through the copper wire and keep your drain line from freezing up.  There is a 90 degree bend in the drain line which makes it difficult to feed the copper wire through.  I suggest using guitar string or picture hanging wire as a snake.  Feed that through first, then us it to pull the copper wire through.  Good Luck!