I once had an electrical outlet with a light switch next to it, in a bathroom. When I replaced the outlet with a GFCI, the light switch quit working.
When you have a GFCI and a light switch is involved, you have to wire things a bit differently so it all works.
Here’s how I fixed it, using a length of wire (use black, or if you only have white wire, put some electrical tape on it) and a wire nut.
Let’s get this out of the way first: If you’re working with electricity, turn the power off at the breaker. And if you don’t have a way to verify that power is indeed off at that circuit and none of the wires are live, throw the main breaker at the house. I know what 110 volts feels like. If you don’t, I don’t want you to learn on account of me.
The original outlet had a white wire and a black wire on one set of posts, just like you would expect. Then there was a black wire connected to the other brass screw on the outlet, which was then feeding the light switch. The other silver screw on the outlet was empty. When I wired the GFCI like that, it wasn’t happy and nothing worked.
Here’s how I fixed it. I removed the GFCI temporarily. Next I took the black wire that had been feeding the outlet and connected it to a short length of black wire, along with the black wire that went to the switch, securing it with a wire nut. This is called a pigtail.
Then I connected the white wire to the silver screw on the GFCI. I connected the new black wire to the brass screw on the GFCI. What about the black wire on the light switch? I had already connected it to the wire nut.
After I did all that, I secured the GFCI back to the electrical box with screws. I installed a new cover plate, and restored the power. The light switch worked right. So did the GFCI.
That’s how you connect a light switch to a GFCI outlet. Like I always say about electricity, it isn’t always easy. But if you work with the power off, you can usually figure it out. Someone who knows less than you figured it out once. That means there’s a very good chance you can figure it out too.