About an hour into deep cleaning a refrigerator for the new year (a GE GFSS6KEXCSS), I was greeted with a loud pop, a bright bulb flash, a smoke smell, and no-longer-functional fridge lights. At first I’d assumed that a light bulb burned out, but this fridge has two bulbs and both were out. Swapping the bulb didn’t help, and the fridge bulbs worked fine in a lamp.

Putting a multimeter on the bulb socket, I’m only seeing 10 Vac instead of 120 Vac as you’d want for those bulbs. Looking online, I couldn’t find anyone that seemed to have this problem. Most of the guides talk about replacing a faulty switch, or control board. But the door seemed to have a good click, and I’d smelled smoke inside the fridge immediately after the pop, so it seemed more likely that the problem was in the fridge.

Looking around, I was able to find a service manual from servicemanuals.net. The website is sketchy, but it did have a real service manual for the fridge.

While the service manual has a lot of useful information about this refrigerator, I think that my model might have been the super budget version because I don’t have a water dispenser, and the LED lights it references are nowhere to be seen. Nevertheless, it did show how you’d go about removing the lighting assembly.

Picture with removing lighting assembly

Two screws and an unplugged wiring harness and the whole assembly was out of the fridge.

Removed lighting assembly

Putting a multimeter on the harness and I can see 120 Vac is available to the assembly, so the causes is probably internal to that. The wiring looks pretty straightforward, but there’s a black cylinder in the middle of it that is probably at fault.

Mystery Cylinder

Looking at the GE Appliances Parts Store, part 507 is labeled as “Bimetal” and “WR02X12808” (Part 507). It doesn’t exactly match what I see, but seems close enough.

Lighting parts diagram

Presumably this is a bimetallic strip used as a temperature regulator. For an appliance bulb that gets hot, it would make sense to have a safety in there to cut the bulbs if they aren’t turned off, since a stuck relay on the control board might cause a fire otherwise. In my case, it probably did get hot enough to trigger, but rather than temporarily open the circuit, it got stuck.

Measuring the resistance of the part with a multimeter, I see it at 140mΩ. Looking at the part itself, it was marked with “60/40”, “ICE”, “062509”, and “BT-121-M” (though the ‘B’ is so faint that it looked like an ‘E’ initially).

For a part number, the BT one seemed most likely, and sure enough, there’s a datasheet that matches the part. The BT-121-M datasheet says it’s not supposed to ever be above 100mΩ, so it’s clearly out of spec.

From what I can gather, the normal mode of operation for this thing is to be closed unless it gets too warm, in which case it will open up. It’s usually used in a refrigerator or freezer for automatic defrost (cut the defroster if it gets too hot), but I can see how a product designer would reuse it here to reduce the number of parts needing to be sourced.

The datasheet says that it should be nominally closed below 40°C/104°F and nominally open above 60°C/140°F, which explains the 60/40. Given that some of the plastic seems to have deformed a little bit above this sensor, I’m not sure if it did its job, but I’d prefer not to try to redesign it.

While I’d normally just buy the part and be done with it, it’s backordered, and I’d like to have a fridge light sooner. A little bit of searching online, and it seems clear that that the Samsung DA47-00243K uses the BT-121-M, but to control a defroster. It doesn’t hurt that the Samsung part can be had for $7.50 compared to the $90 that the GE part costs.

With that in mind, I pulled the thermistor out and temporarily bypassed it to validate my understanding. Sure enough, I can see 120Vac in the socket again with the fridge open, and a fresh set of LED bulbs light up. I checked to make sure that the bulbs go off when the door is ever so cracked, and it goes off. Once the new BT-121-M shows up, I’ll go ahead and replace it on the assembly and be done.

Working light