Replacing the GFCI in the garage will determine if that's the culprit receptacle. If replacing it fixes the problem, then that was it--but you didn't need me to tell you that! If the problem remains, try replacing the outdoor receptacles first, one-at-a-time if you're looking to save money. If the bubble covers on the outdoor GFCI's have a leaky gasket, that could be the issue. Water is the worst enemy of the GFCI receptacle.
Only trial and error will answer your question, but if you're feeling sporty (and I mean if you have the requisite experience and common sense to perform this electrical work), you can simply shut the GFCI breaker in the garage, and disconnect the outdoor receptacles on that circuit. If either or any of the outside receptacles are the issue, that'll correct the problem until you feel like replacing the receptacles. If you go this route, be sure to place a wirenut on the ends of the wires you remove from the receptacles to "safe them off."
As Tangster said; if a staple or screw was shorting into the conductors on the affected circuit, it could take time before sufficient rust causes the short to manifest itself.
To review, I'd first replace the suspect GFCI receptacle in the garage hosting the sump and return pump. If that doesn't do it, I'd just take the outdoor GFCI receptacles off-line by simply disconnecting them all. You'll probably have your answer. If not, post again and we'll move on.
Good luck!! Be careful--make sure the circuit is off. Plug a radio or lamp into the receptacles in question to be very sure they're dead before working on them.
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90G display tank. Kent Phos Reactor running carbon and ROWAPhos, Coralife 225 Skimmer. Wet/Dry converted to 10 Gal Sump w/ auto topoff float valve connected to Typhoon 5 Stage RO/DI. Little Giant inline (external) return pump. Coralife 2x150MH, 14K + 2x96W PC Actinics, Ecotech Vortech propeller pump, Hydor Koralia #2. |