AEDs continually monitor themselves through automatic self-tests.
These built-in tests check internal electronics, monitor battery status, and identify some hardware faults. Self-tests are an automated internal process, and if faults are identified, they are generally displayed through visual icons or indicator lights, and/or through audible prompts.
While these built-in checks are valuable, they cannot confirm every aspect of the device's real world performance.
Self-test can only confirm the device detects faults within its own systems, it can't verify every aspect of real-world performance. A status indicator light is not a guarantee that the AED will shock. That means self-tests alone aren't enough to guarantee your AED will work when it's needed.
For the highest level of assurance, Heart Saver recommends every AED undergoes Annual Performance Verification.