LG Refrigerator Weird Error Code E ru

Hi gang-

I have an LG fridge, model number LMXC23746s. About every 8-10 months, the fan behind the freezer panel starts making noise and the usual remedy is to defrost it. Happened again a few days ago and this time I decided to do some detective work. I checked the defrost heater and it measured a resistance that is within specification. I also checked both freezer and defrost temperature sensors and both checked out good resistance-wise with the values listed in the service manual. Finally, I tested both ice maker and freezer fans and both work fine, revolve without noise or resistance, etc. Family started to get a but antsy living out of coolers (and I was tired of chasing bags of ice) so I told my wife that I'd just restored everything and then do more troubleshooting the next time things went south.

Put everything back together today and all seemed fine. I checked for any new error codes a couple of times after restarting and got none. After a while, I pressed the FREEZER and ICE PLUS buttons again and the display read F and ru. Weird because this code is not listed in the service manual nor can I find anything about it online (save for one post from several years ago that went without a solution). So far, the refrigerator and freezer are both cooling nicely and the fridge is making a nice quiet purring sound.

Anyone have a clue as to what this means?

Thanks!

J