APC Pro 1500 S Stuck in F02 Error Mode with Constant Beeping

[lazywarp]

I have an APC Pro 1500 S that randomly tripped while I was at work and stayed beeping for hours (my dog is now traumatized to the beeping sound).

No matter what I do the unit only shows F02 with a constant beeping. I've replaced the battery, let the whole unit discharge without the battery connected, and several other ideas I found online.

The unit is out of warranty.

[Teken]

The F02 code indicates there is a short connected to the units outputs. Remove AC Mains from the wall outlet. Disconnect all connected loads from the rear of the unit.

Disconnect the battery and than press and hold the power button. You may hear a click / beep when this is done. You may not hear anything at all either. ☝️

Reconnect the battery and apply AC Mains to the unit and power on. How does the unit behave now without ANYTHING attached to the outputs?