This page serves as a starting point for troubleshooting general errors with your TurboGrafx-16.
Controllers Not Working
If your controller is not working properly, such as some button presses not working, check if another controller is having the same issue.
If all controllers experience the same issues, it is likely an issue with the console motherboard. Disassemble the console and reflow the solder on the controller port's pins. Carefully inspect the board for any cracks that may have severed traces, as the area of the motherboard housing the controller connector is thin and prone to cracking if the console is dropped.
If the issue seems to be with one controller, disassemble the controller, and test continuity for the controller wire to verify that it doesn't have a break. If it seems okay, remove the rubber button membranes and gently smear each button's dark side against a piece of paper in one direction. Then, use Deoxit F100 — which is specially formulated for conductive plastics — to spray the button contacts on the controller PCB.
If you believe the contacts are the issue and cleaning them doesn't help, you can order new NES controller membranes which can be used as a direct replacement for all but the Run and Select buttons.
In addition, if the wipers for the turbo switches are not making good contact with the board, they can prevent the I and II buttons from working. The wipers can be polished with a fiberglass pen, then coated with Deoxit F100 to prevent further corrosion or oxidation. The wipers can also be carefully bent back slightly so they can make better contact.
Capacitors
Almost all of the "Duo" consoles suffer from failing capacitors. It's strongly recommended that you do a full cap replacement on all Duo consoles, as even if it's still working, leaking caps could be slowly damaging your motherboard! Unless you've done something like this before, it's recommended having one of the above modding services do it for you, as it's easy to damage your console.
Jason from Game-Tech has a video that shows a cool trick to make the cap replacement easier: