This is condo constructed in 2006. When I moved in (2017) everything worked fine (unless I simply didn't notice?), lights were incandescent.

In 2019 I changed the master bathroom light fixture from Incandescent to a new LED one, and installed a can light in the shower, piggy backed from the wiring near the new fixture. This was done with help from an electrician friend, I am pretty confident that everything here was proper.

Later that year, I changed the common bathroom light fixture to the same LED fixture.

Last year my GF moved in, which was the first time I've had more than just myself at home. She told me that she has been noticing weird things with the the light in the common bathroom.

Both bathrooms have two simple switches, one for light and one for exhaust fan. The bathrooms have a shared wall, here is the layout: https://i.imgur.com/PaXxqPT.png

So here's what happens:

If the common bathroom light is ON first:

Turning the common bathroom fan from ON to OFF causes a 1-second flicker OFF in the common bathroom light.
Turning the master bathroom light from ON to OFF does NOT cause any flicker.
Turning the master bathroom fan from ON to OFF causes a 1-second flicker OFF in the common bathroom light.

Can you maybe guess what's happening here? What should I look for to find the issue?


To me it sounds like a bad connection usually a back stab.

the inrush of power to the just turned on device is causing a low voltage that is visible at the device that is on.

Both are on the same circuit and the problem is closer to the junction box.

When we wire this way everything is daisy chained 1 weak connection on the hot or neutral wire prior to the point we’re both feed from will be the point to start looking then closer to the service panel or breaker box.

These problems don’t usually get better but worse over time so you may loose both lights at some point if the loose connection is not found and fixed.

If a back stab sometimes moving a switch or receptacle can fix the loose connection (until a large load is applied then it will usually fail again if additional receptacles are on this circuit.

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