Engineering guide
Liebert APM2 fault LEDs and alarms
On a Liebert APM2, each power module has a green Run LED and a red Fault LED: steady red means a module fault, flashing red an abnormal condition such as abnormal input or a module overload. Vertiv's manual SL-71948 also explains that silencing an alarm does not clear it, and that after a fault shutdown the cause is cleared before Reset Fault is used.
Key facts
- Battery low pre-warning time: 2 to 60 minutes, 5 minutes by default
- End-of-discharge voltage: settable from 1.60 to 1.85 V per cell (VRLA)
- DC bus capacitor discharge: wait 30 minutes
- Rectifier restart after overtemperature: five minutes after the signal disappears
The two LEDs on each power module
Each APM2 power module carries a green Run LED and a red Fault LED. Vertiv's user manual, SL-71948, gives their meaning:
| LED | State | What Vertiv's manual says |
|---|---|---|
| Run (green) | Flashing | The inverter is starting but has no output yet. |
| Run (green) | Steady | The inverter is supplying power. |
| Fault (red) | Steady | A module fault, for example an auxiliary power failure, rectifier overtemperature or failure, battery converter failure, soft start failure, abnormal input current, inverter failure or output short circuit. |
| Fault (red) | Flashing | An abnormal condition, for example a charger failure, abnormal input voltage or frequency, input undervoltage or reversed phase, battery unavailable or reversed, current sharing failure or module overload. |
| Fault (red) | Off | None of those failures or alarms. |
A steady red Fault LED on one module of a redundant system means that module needs work while the others carry the load; the touchscreen alarm text tells which fault it is.
Silencing an alarm
The Silence button on the touchscreen stops the audible alarm. Vertiv's manual notes that how long it stays silent depends on the UPS type, the alarm and the configuration, and that the alarm message stays on the display until the condition is corrected. Silencing is not clearing.
Battery warnings and the end of discharge
During an outage, the APM2 warns before the battery reaches its end-of-discharge voltage. The manual gives a battery low pre-warning time adjustable between 2 and 60 minutes, 5 minutes by default, and an end-of-discharge voltage settable from 1.60 to 1.85 V per cell on VRLA batteries. At that point the battery converter shuts down to protect the battery.
After an emergency power off or a fault shutdown
Vertiv's procedure is to clear the cause shown by the alarm message first, then use Reset Fault and restart. If the shutdown came from overtemperature, the rectifier restarts by itself five minutes after the overtemperature signal disappears. Two safety points from the same manual: the battery terminals still carry hazardous voltage after a complete shutdown, and the internal DC bus capacitors need 30 minutes to discharge.
Alarm contacts for the building system
With the optional IntelliSlot IS-Relay card, the APM2 reports to a building or monitoring system through dry contacts:
- Low Battery: closes when the low battery point is reached.
- UPS Fault: closes when a UPS fault occurs.
- On Battery: closes when the UPS runs on battery after a utility failure.
- Summary Alarm: closed while no alarm condition is present, so it opens on any alarm.
When to call
A steady red Fault LED, a module that will not return to Run, or a battery warning that comes early in every outage are reasons to book a visit. Battery strings are the most common repair, handled through our battery repair and replacement service; for the module side, the guide to modular systems explains why one failed module does not stop the others.
Related pages
Common questions
What does a steady red Fault LED mean on an APM2 module?
A module fault. Vertiv's manual lists causes such as an auxiliary power failure, rectifier overtemperature or failure, battery converter failure, soft start failure, inverter failure or an output short circuit. The touchscreen message names the fault.
What does a flashing red Fault LED mean?
An abnormal condition rather than a module failure, for example abnormal input voltage or frequency, a reversed phase, battery unavailable, a current sharing failure or a module overload.
I silenced the alarm. Why is the message still there?
Vertiv's manual explains that the alarm message stays displayed until the condition is corrected. Silencing only stops the sound.
How do I restart an APM2 after an emergency power off?
Clear the cause shown by the alarm message first, then use Reset Fault and restart following the manual. If you are unsure of the cause, call a technician before restarting.
How long is the battery warning before shutdown?
The battery low pre-warning time is adjustable from 2 to 60 minutes and is 5 minutes by default, according to Vertiv's manual.
Sources, checked October 4, 2026:
- Vertiv, Liebert APM2 user manual SL-71948, pp. 18, 33, 78, 108, 113, 147
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