I am going to share my story of how I fixed my Bosch KGN53X00AU. It had worked well for about 5 years until one day a layer of ice started forming at the bottom of the freezer compartment. After breaking and cleaning the ice for several weeks I started looking for information of forums. Defrosting did not help. Customer support line was totally useless.
Finally I decided to fix it myself. Nothing was wrong inside, so I decided to check the drain pipe at the back of the fridge.
Before going into the details I would like to remind you where the water comes from. If we look at how a fridge works (http://faculty.wcas.northwestern.edu/~in...) we can see two heat exchangers internal (blue) and external (red):
Water from the air freezes on the internal heat exchanger and can forms a thick layer of ice. Back in the old days people had to defrost their fridge regularly to remove that ice. Modern fridges do it automatically. The internal heat exchanger has a heating inside which is periodically turned on by the controller during defrost cycle.
The ice melts and the water runs down a special drain pipe outside of the fridge camera into a special tray at the back of the fridge. Usually they put this tray on top of the external heat exchanger (red one on the picture). This exchanger is hot, so the water quickly evaporates back into the room.
If the drain pipe is blocked the water leaks where it can. I my case it leaked into the freezer camera and got frozen at the bottom. If the pipe is blocked with frozen water defrosting may help. Unfortunately, something else was blocking my pipe.
First of all I looked at the back of my fridge:
That plastic grey panel is easy to remove by pulling it from the sides.
Now we can see the black pump on the right and heat exchanger with the plastic water tray on top of it on the left.
If you look closer you will see a plastic disk inside that tray. The drain pipe was inside that disk, but I could not remove the disc because there was not enough space. In order to remove it I had to lay the fridge on the floor (be very careful not to drop it) and unscrew the plank that holds the heat exchanger.
That gave me enough room to remove the plastic disk:
The drain pipe can be seen here right above the tray:
The disk is a water seal and the problem was right in it. It was clogged with some jelly like substance, so the water could not run through it. I cleaned it up:
After assembling all the parts back the fridge works like new.