Have a Opto LED switch fail?
If a wire lead is not broken off, then 80% of the time either this Detector, or the Emitter has packed it in, as they breakdown over time, and just quit.
No need to purchase the whole assembly, but they are offered here too, if de-soldering, soldering is not your thing.
Here is a set, change both. Not sure which is which?
Detector will be marked with a “Q” on the board or in the schematics, and the Emitter a “D1”.
Watch for the “Flat” edge on the round base, that’s how you align the part to solder in place.
Last photo shows one type of assembly these are found in.