MJR wrote:
symptoms
1. garage lights won't turn on
2. garage door opener won't go up/down
3. when light switch is off, garage opener sensors light up
4. when light switch is on (lights don't go on), sensors go off
5. Voltage tester shows 104 volts in all outlets in garage


what should I do (besides calling an electrician)?

the problem began soon after the first rains in 6 weeks

the power to the garage is underground conduit from the house, under
the yard to the garage.


10 to 1 you have a splice or screw connection which has gone bad in
either the hot or neutral feed wires to the garage. It is not completely
open, but has unusably high resistance.

At the very low current drawn by the garage door sensors there's still
enough voltage (the 104 volts you measured) to let the sensors light up.
Any load much heavier than that causes the voltage to collapse to near
nothing. That's why trying to turn on a light makes the sensors go out.

Good luck finding it. If you are lucky and there are no splices or
joints in the wires underground you may just find that there's a loose
screw connection or wirenut on something like the circuit breaker
feeding that line or maybe in a box in the garage where the underground
wires split out to the various things using power in the garage.

Try using a light bulb with an appropriate socket and leads instead of
your voltage tester. That'll give you a more realistic load to test with.

HTH,

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."