I have a panel circuit with a 20A breaker. This circuit has 4 regular outlets and 1 20A GFCI outlet wired at the end of the circuit. The GFCI outlet is 15 years old and is an outside receptacle with the rest of the circuit is located inside my garage. The GFCI outlet has 1 wire with a Black and White conductor (+Grd). While running an extension cord to this GFCI outlet to temporarily run my Pool Pump, the GFCI tripped (Faulty Extension cord confirmed). The entire series of outlets also lost power although the Panel breaker (20A) did not trip. The GFCI will not reset either. (Diagram included) Will replacing the GFCI correct my no power issue for the entire circuit?
No, it will not.
In fact, there’s probably nothing wrong with the GFCI.
What happened is you temporarily overloaded the circuit due to said munged extension cord, and this overload blew a fuse in one of the other receptacles.
OK, “fuse” is the wrong word, obviously receptacles do not have fuses... but it fails exactly the same way. One of the receptacles has a poor connection. The overload of the extension cord burned the connection out.
Find the bad connection
The most likely cause is “back-stab” connections - where the wire is jabbed into a hole in the back of the recep. (Mind you those should never be used on a 20A circuit). It can also happen from a loose screw connection.
Go to each recep, identify any backstabs, forcibly pull and twist them out, and search them for burn or spalling marks. If you find one, gotcha! Move that wire to the side screw by stripping a little more insulation off and shaping it into a J-hook to go around the screw. Set it clockwise so the screw tightens not spreads the wire.
Then move the GFCI so it does protect the downline
The sockets on a GFCI are on the LOAD side of the GFCI - they are in the protected zone. You can put other connections in the protected zone; just attach them to the LOAD terminals of the GFCI. That is the only thing LOAD should ever be used for; all other connections should be to LINE.
So, relocate this GFCI recep to one of the indoor outlets. That will get it out of the weather and make it last a lot longer. The closer to the panel it is, the more it will protect - but don’t allow a refrigerator or freezer, fire alarm, sump pump or radon pump to be on GFCI protection, as they are safety devices. Other than that, garage outlets should be on GFCI if possible.