Topic: I got a very strange controller issue.

Console: Nintendo Switch
Controller the problem is on: Red Joycon

So the controller all works fine and you can use it. But the issue is the green lights don't turn on and you can't use it to connect it for an other player.

(Example: Mario Kart you want to play three player but you can't as the red controller is not connecting as the green lights won't turn on)

Anyone know how to fix this?

Have you held the little button on the rail? (Near the SL and SR buttons)

That should reactivate it.

@Dr_dreads So, the joy-con buttons register on the switch, but you have no green lights? That may suggest a defect? In my experience, the only time lights don't come on is when the joycon is dead & nothing works until it's got some charge again. (just an observation. I don't know if that's the only reason lights don't come on).

For the sake of problem solving, make sure MK8 is running, and you've selected multiplayer. (I even picked 2player and the third joycon-half still paired as a third controller.) Make sure your controllers are current with their software updates. You have to manually run the update for each controller. Go to: System Settings/Controllers and Sensors/Update Controllers. Controllers must be plugged in via usb cable to update. (forgive me if you already updated controllers)

I combed through Nintendo support and couldn't find your issue(pasted link) If you're stuck after all this, I myself would call Nintendo Support. (In the US here they're good! and quite pleasant).
https://en-americas-support.nintendo.com/app/answers/list/st/...

Also the Switch will not sync a controller if you are playing a 2 player game, and you are trying to sync a third controller. I would close out of Mario Kart and go to the main screen and try to sync it there just to determine if it will sync at all. If it won't I would call Nintendo.

@Dr_dreads Do the SL and SR buttons work?

I suspect you've got the issue I had that is apparently fairly common. The ribbon cable inside the joy-con that connects the rail LEDs and SL/SR buttons is subject to kinking to such an extent that it breaks.

You can DIY repair it (if you have a tri-wing screwdriver) and a replacement ribbon cable can be had on eBay for less than £2, if you're prepared to wait for delivery from Asia.