Electrical

UK Two-way switch problem - light only works when both switches are ON

Freddie P 15/10/2026 - 1.11 PM

I'm trying to fix a lighting circuit in my house (UK) where two switches on different floors control the same light. Currently, the light only turns on when both switches are in one particular position. I'd like conventional two-way switching, where flipping either switch always toggles the light. Existing wiring Both switches have COM, L1 and L2 terminals. Switch A (single switch, one floor): - COM: Red wire (going upwards) - L1: Red wire (coming from below) - L2: Black wire Switch B (two-gang switch, other floor): The left gang controls the problematic light; the right gang controls another light independently. - Left COM: Red wire linked to L1 of the right gang - Left L1: Black wire - Left L2: Red wire The red and black wires on the left gang appear to enter the box through different cable sheaths. What I've tested 1. Both switches appear to operate correctly when checking continuity between COM and L1/L2 in their two positions. 2. With the power isolated, I tried joining each possible pair of conductors at the single switch and measuring continuity between COM, L1 and L2 at the double switch. All combinations returned OL. The conductors at the double switch were not individually isolated, so these results may be inconclusive. 3. I temporarily swapped COM and L1 (both red) at the single switch. Interestingly, this made the single switch determine which gang of the double switch could control a light. I've since restored the original wiring. 4. In the original configuration, the right gang operates its separate light independently, while the problematic light only turns on when both relevant switches are in position 1. The ceiling light fitting itself only has live and neutral connections visible, so it doesn't provide much information about the switching arrangement. My questions - Does this sound like an incorrectly wired two-way circuit, or two switches effectively connected in series? - What additional continuity measurements would help identify the wiring arrangement without live testing? - Is there a likely straightforward correction using the existing wiring, or might an additional conductor be required? I'd prefer to retain ordinary mechanical rocker switches rather than install smart switches if possible. I have photos of both switch boxes and can provide them. I'm looking to understand the fault before having any wiring altered. Thanks for any advice!