An outdoor GFCI that's under a covered porch has been working fine for a long time, but recently started failing to turn off when the 'test' button is hit. Here's what I observed.
Right after plugging something into the outdoor GFCI outlet, the GFCI and a light that gets power from it both turned off. I realized I may have accidentally hit the 'test' button on the GFCI while plugging this bulky plug in, which has happened before. I pressed reset and as expected, the light and outlet both got power back, but once power was back I could here an electric fizzling sound. I pressed reset again and firmer, and while depressed the power was off and when released the power and fizzling sound came back. I pressed test intentionally to see if that was working, and it did cut the power. Upon pressing reset a few moments after that, the power and fizzling sound came back, and then after a second or two a sudden pop and flash of blue from within the outlet, as if something arced. Power stayed on, and now when I press 'test' nothing happens and when I press 'reset' power is off when depressed and on when released. The outlet seems to function normal aside from 'test' not cutting power (and the test button itself appears half-depressed and not being fully released).
I'm wondering what could've happened, and what to watch out for if opening this up. Will simply replacing the GFCI be good enough, or should other follow up measures be taken to ensure safe functioning?