It's a built-in unit with the compressor on top. The drain pan is getting full and overflowing. I've started draining it once a month and that's often enough to stop it from overflowing. I think the problem is that the water in the drain pan is not evaporating, but maybe it could be that there is just too much water getting in there for the evaporation to keep up.

So, question #1, how quickly would/should the pan fill up without any evaporation?

My attempts at diagnosis have been based on the guess that a lack of evaporation is the cause.

I'm not very familiar with refrigerators/freezers, but I know how to work a multimeter, understand basic circuitry, etc.

As best I can tell this unit has four components associated with the drain pan:
- The pan itself
- A float switch
- A service fan that blows air across the drain pan
- A pan heater

When I went to diagnose it today the service fan wasn't spinning, and didn't kick on when I triggered the float switch, so I thought that was likely the problem, I pulled the fan out. It's a 24VDC fan, so I connected it to 3 somewhat discharged 9V batteries and it spun up like a champ, so it's not spinning on the fridge because it's not getting power. So, question #2, how often should that fan be spinning?

I checked the float switch as well and it seems to be working properly (resistance across the leads drop to 0 when I lift the float).

I'm not sure how to check the pan heater, other than maybe to somehow lock the float switch up and wait to see if it heats up. Question #3, how can I test the heater?

Question #4, what contributes more to the evaporation, the fan or the heater?

Thanks to anyone who reads this and/or offers assistance, much appreciated.