Light switch not working?

The problem I have is I replaced my sons 1-way light switch (plastic) last week and now the light in our bedroom doesn't work properly. My sons light switch has 4 red wires, 2 going into com and 2 going into L1. When my sons light is switched off my bedroom light doesn't work at all, but when my sons light is switched on I can turn my bedroom light on and off. I checked my bedroom light switch (plastic) which hadn't been touched and found 2 red wires 1 in Com and 1 in L1, but there were also 2 other wires a green yellow wire which wasn't connected to anything and a black wire which had been cut very short and was also not connected. I would be very grateful of any help regarding this matter.

Regards Mark.

Are you absolutely sure that you have connected the new switch in exactly the same way the old was connected? If you had then nothing would have changed. What connections ae there on the old switch? What you have is your bedroom swith connected in series with your sons. It's like having 2 taps on a water pipe, if either is off no water flows. Are your and your son's switches located close together, maybe on opposite sides of the same wall?

The first thing you need to do is to sort out what the wires in your sons switch are doing - and for that you'll need a simple multi-meter. You will have at least one which is a permanent live. Now, it may be that whoever put the lights in is using this permanent live to supply the power to your light as well., in which case you may well find this is the only live in that switch. You have to do this with the power on so be very careful. Disconnect all the wires (with the power off) and spread them out so they are touching nothing, then test with the power on and see what you have got. At that point put the result an here and we'll go from there.

Didn't you note what went where to start with?

It sounds like the lighting circuits loops in at the switch in your sons bedroom. You will have a 3-1 split of red cores. The one on its own goes to son's bedroom light. I cant think of any other way it would have been done.

2 reds feed other lights (your bedroom ), the other 1 is permenant live feed for all the lights, so you can see that these 3 must be connected.

The best way to do this is as follows: (with the offending switch OFF)

Turn the electric off for safety, and pull the switch off the wall so you can access the terminals, but all wires are still connected. Turn the electric back on, make sure switch is still off.

Test which terminal is live and which is dead (with a multimeter set on volts AC, or if you must, a neon screwdriver).

Then turn the main electric off. ONE of the cores in the dead terminal must be swapped to the live terminal. Try one, see if it works, if not, try the other. This should cure the problem. If not, get back to us.