DIY Doctor UK DIY Forums

Replacing a bathroom PIR switch with a cord pull switch. The switch controls the light and the extractor fan. Two cables coming from the ceiling at the switch with two core and earth in each. I put the two live red wires, one from each cable, into the COM terminal and the two black neutral wires into L1. Turn on the power again at the consumer unit and the light and fan come on which is fine but operating the switch to turn them off trips the circuit. Same happens if I put both neutrals into L2.

What am I doing wrong and what's the solution?

You have answered your own question.
You said put the lives and the neutrals in the switch. You never connect live to neutral as that is a dead short.

Without having a meter to establish how it has been wired I would say:
1 red in com
1 red in L1

The two blacks in their own piece of connector strip, NOT in any terminal on the switch.