GFCI outlet not tripping when tested with the test button

[Venkman]

Electrical outlet #1 is a GFCI duplex outlet. there are 3 additional downstream outlets on the circuit.
When I test the GFCI outlet, using the test button on the outlet, the downstream outlets lose power but the GFCI outlet stays powered.
The outlets are 28 years old.

What is the most likely cause and solution to the GFCI not functioning correctly when tested? I'm thinking that the GFCI has failed internally.

[machinist]

Hi, Venkman. It sounds like your GFCI outlet is likely worn out. Normally, when you press the test button, it should shut off power to both itself and any downstream outlets on the same circuit. Since your GFCI isn’t losing power (only the downstream outlets are), it’s probably failed internally—especially given that it’s 28 years old.

A new GFCI should restore full function. Once it’s swapped out, test it again to make sure both the GFCI and the downstream outlets lose power when you press the test button. In rare cases, incorrect wiring can also cause unusual GFCI behavior. When installing the new GFCI, double-check that the wiring is securely connected to the correct terminals (the "line" and "load" sides). This ensures it’s wired to protect both itself and the downstream outlets.

A new outlet should fix the issue, and a quick check on the wiring will ensure everything’s set up safely and correctly!