The most common issue is the Replace Battery alarm, which appears when the internal battery fails a self-test or reaches the end of its service life. This is expected behaviour after 2 to 4 years and is resolved by installing a genuine APC replacement battery cartridge. Other frequent issues include the Site Wiring Fault alarm, which indicates a problem with the building wiring rather than the UPS itself and requires a licensed electrician to investigate. Overload alarms occur when connected equipment draws more power than the UPS is rated to supply. Unplanned bypass mode usually results from an overload or internal fault. Fan faults and internal fault codes such as P.13 require a service technician. Most day-to-day issues can be diagnosed using the front panel LCD and the error log accessible through the menu.