Hi!

What is the likely
problem and what's the part to check?

Some of these motion sensors use a relay to switch the attached load on and
off. You might check to see if the relay was perhaps undersized to the work
at hand. The contacts could have welded themselves into the "on" position
even while the sensor is working fine.

There are some that are completely silent when switching. I assume these
might use something like an SCR or triac to do the switching...but I don't
know as I've never had one apart. I'm not sure an SCR can even be used for
the purpose of switching something on and off.

William