If the switch box is metal it also must be pigtailed to the grounding wires. Touch the probes to the 3 wires removed from the switch until you get a reading on the tester indicating continuity.

Identify Wires For New Doorbell
3 way switch 2 red wires. Red wire power or hot wire black wire power or hot wire white wire neutral bare copper ground. Now go the other switch location. The red and black wires travelers of the 3 wire cable get connected to the common terminals on the three way switch. When wiring a 3 way switch circuit all we want to do is to control the black wire hot wire to turn on and off the load from 2 different locations. I have a 3 way switch with 2 red wires and 2 black wires and a ground and trying to figure out which wire goes where. It could be there is or once was a three way switch in play.
The grounding wire is connected both to the switch using a pigtail wire and to the second cable run passing onward to the next switch. At the line switch box the black wire of the two wire line cable gets connected to the copper or black screw terminal on the three way switch. None are hot unless the other light switch is turned on in which one of the reds is hot. The black and red wires are travelers and are connected to the. At the switch where you found power in step 2 put the other 2 wires you removed from the switch together and wire nut them. Or it could be the red wire is the wire going to the fixture and the black wires are the live wires.
Once youre done youll be able to control a light from two switches. Wiring a 3 way light switch is certainly more. 3 way switch 2 3 way wire which has black white and red coated wires and of course a copper ground wire. The cable run linking the two switches is made with 3 wire cable. Set the electrical tester to the continuity or ohms setting. Black wire power or hot wire white wire neutral bare copper ground.
This story features diagrams that show how to wire 3 way switches. When you see a red wire in a light switch box it can mean two things. Well cover the easier scenario first. While wiring a 3 way switch is more involved than something simple like installing an electrical box for an electric fireplace the task can be simple enough if you follow along with these instructions. I tested the other wires on the hot red to see which turn the light on and thats one of the black wires. The wiring is more complicated than a traditional single pole switch but well explain how to make the connections.
It doesnt matter which traveler goes on which common terminal.