Here's the deal. I bought a refurbished 3DS XL system at GameStop on Black Friday. It was meant to be given to my brother as his Christmas gift today. I also ordered a Circle Pad Pro XL from the Nintendo Online Store for him to use with games like MH4U. I never thought to test the CirclePad Pro XL before today, because I trust Nintendo's techs. That said, when he went to use it today, he got a surprise. The game says it can't connect to it.
I immediately began troubleshooting. I checked MH4U's options, switched in known good batteries, etc. I switched to Majora's Mask 3D and it couldn't connect either. I figured this meant there was a problem with either the CCP XL or the 3DS XL. So I took the 3DS XL to my room and turned on my iMac camera, which can see infrared light. I've used this to troubleshoot Wii Sensor Bar issues in the past, and sure enough, I could see a light blinking in my brother's 3DS XL IR port. I could not see any light coming from his CCP XL, so I looked at my regular CCP and couldn't see a light either, even though I knew it worked.
In a last ditch effort to troubleshoot, I put my regular 3DS in his CCP XL, turned on Majora's Mask 3D, and it worked! Yet his 3DS XL still can't connect to it, even though mine can. I'm almost certain this means something is wrong with his system's IR port, but as I can see a light working in it when held in front of my iMac camera, I have no idea how that's possible or what exactly could be wrong. The only other thing I could do to troubleshoot would be to buy an Amiibo Adapter and test it with both 3DS systems, as it uses the same IR port functionality. I have been meaning to get one, but really can't justify the expense right now just to use it as a troubleshooting tool, because I already have a New 3DS XL that can use Amiibo. Does anyone have any ideas?