Let me apologize first for the inconvenience this has caused. The P.16 error also refers to Power Sys Error and is related to either a + or - DC Bus Soft Start. The UPS has detected an error upon start up from online or on battery and the UPS is not able to pre-charge the DC bus.
There are multiple reasons for this alarm, we would have to know what was the condition of the UPS when this alarm was reported for example, upon power up, when the UPS is plugged into an outlet, etc.
For troubleshooting, you can clear the alarm via the display and turn the output On again. If the issue would persist, we then recommend doing a brain-dead on the UPS. If the UPS wouldn't recover from the fault, replacing the UPS would be the recommended action.
I have provided the FAQ article below that explains how to perform the brain-dead.