APC UPS error codes and audible alarms: complete troubleshooting guide

Scope and how to use this guide

This page covers single-phase APC by Schneider Electric UPS units sold in Canada at 120 V: Back-UPS ES (BE, BN), Back-UPS Pro and Back-UPS RS (BR, BN, BX models with an LCD), Smart-UPS C (SMC), Smart-UPS line-interactive (SMT, SMX) and Smart-UPS On-Line (SRT). It explains what the unit is reporting, which actions APC documents as safe for the user, and which conditions need a technician.

Out of scope: the internal repair of any unit, firmware-specific behaviour not documented by APC, and three-phase Symmetra and Galaxy systems, which report text alarms through their own displays and network cards and are covered only in the safety section. We do not publish runtime figures here; runtime depends on the load and battery age and is better estimated with the UPS runtime calculator.

Three assumptions apply throughout. The UPS has a 120 V input. The battery is the original type or a correct replacement cartridge. Nobody has opened the chassis. If any of these is not true, start with the safety section. If the unit is still under manufacturer warranty, contact APC first: third-party repair may affect warranty coverage.

1. Identify how your APC reports faults

Read the full model number on the rear label before looking anything up. It tells you which of the four reporting systems you are dealing with (the prefix alone is not enough: BN is used for both Back-UPS ES and Back-UPS Pro models), and therefore which table on this page applies. Table 1 maps each family to its fault display, using the model lists that APC gives in each troubleshooting document.

2. Back-UPS Pro and Back-UPS RS F codes (F01 to F09)

The numeric F codes belong to the Back-UPS Pro and Back-UPS RS units that have a small LCD. When one appears, the SYSTEM FAULT indicator lights and APC’s instruction is to match the number on the LCD to its code list. APC publishes the list in three troubleshooting documents for these ranges (BR1300G / BR1500G, the BR and BN “MS/MS2/M2” models, and the BX850M to BX1500M). The lists match except for three points: F08 (Fan Fault) appears only in the BR1300G / BR1500G document, F05 is called “Charger Fault” there and “Charge Error” in the other two, and F09 is “Internal Fault” there and “Internal Error” in the other two.

For the last seven rows, the BR/BN and BX documents are explicit: errors F03 to F09 cannot be corrected by the user. That gives a simple triage rule. F01 and F02 are about what is plugged in, so they are worth one careful attempt at the outlets. Every other F code is about the UPS itself.

3. F02 on an APC UPS: on-battery output short

F02 is the code most people search for, and it is widely misreported as a charger fault. On the Back-UPS Pro and RS units that display it, APC defines F02 as On-Battery Output Short: a short circuit on the battery-backed outlets while the unit is running on battery. In our field experience the culprit is usually a connected device, cord or power bar, not the battery; APC’s own corrective action points the same way, since it starts by disconnecting the equipment.

APC documents a second situation that produces the same display. Schneider Electric’s FAQ on F02 and F04 at power-on (models BX1500M, BN1350M2 and BN1500M2) says that if the internal battery has not been correctly connected, the unit may show F02 or F04 when it is switched on. APC documents that case for those three models; the battery connector check below is harmless on other Back-UPS Pro units, but the diagnosis is only documented for them.

What to do with F02, in order

1. Turn the UPS off. Disconnect every device from the Battery Backup outlets; the surge-only outlets are not involved.
2. Turn the UPS on with nothing connected. If the code does not return, turn the UPS off, plug in one device, turn the UPS back on, and repeat for each device. The device whose connection brings F02 back, or its cord or power bar, has the short. Keep it off the UPS until it has been checked.
3. If F02 appears with nothing connected, check that the internal battery connector is pushed fully home. This is APC’s first step in its power-on F02 / F04 FAQ.
4. Following the same FAQ, turn the UPS off, unplug it from the wall and try switching it on again from the battery alone.
5. If the unit still shows F02, stop. APC’s instruction at that point is to contact support with the model number, the serial number and a video of the display during start-up. Do not keep a UPS that shows an output short at start-up in front of equipment you care about.

4. F01, F05 and the codes you cannot clear

F01, On-Battery Overload. The load on the battery outlets exceeded what the UPS can carry on battery. APC’s action is to turn the unit off, move non-essential equipment off the Battery Backup outlets, and turn it back on. In our experience, laser printers, space heaters and power bars daisy-chained into the battery outlets are frequent causes. If F01 returns with a modest load, compare the load with the unit’s rating; it may simply be under-sized for the job.

F05, Charger Fault or Charge Error. This, not F02, is the code APC names as a charging fault, and APC classes it as one the user cannot correct. Contact support before buying a battery: a new battery is not a documented fix for F05.

F03, F04, F06, F07, F08 and F09. Xcap overload, clamp short, relay welding, temperature, fan fault and internal fault all describe the condition of the UPS itself. APC’s position is the same for all of them: contact technical support. Two GDF field notes follow from that. First, with F07 (temperature) it is worth checking the room before calling: a unit pushed against a wall or buried in a closed cabinet may simply be hot. Second, F06 indicates a welded relay and APC gives no user action for it, so take the unit out of service rather than restarting it repeatedly.

For a Back-UPS Pro, the repair-or-replace decision after an internal code is usually an economic one. For a Smart-UPS or anything protecting regulated equipment, a documented assessment is the better route; our APC UPS maintenance and repair service covers both.

5. Smart-UPS C: what L.01 and L.02 mean

On Smart-UPS C models (SMC), the codes L.01 and L.02 are battery end-of-life alarms, not communication faults. Schneider Electric’s FAQ for all SMC models states that L.01 means the battery is near end of life and L.02 means it has exceeded end of life. APC’s instruction for both codes is to acknowledge the alarm with a short press of MUTE (about 0.2 second) and contact technical support to arrange the battery replacement as soon as possible.

If an SMC shows a code that is not in this table, the FAQs cited here do not define it, so we do not publish a meaning. Note the exact code, the model and the serial number before calling support; that is what any technician will ask for first. In a clinic or other regulated site, have the replacement done and logged by qualified staff or your service provider, then run a self-test.

6. Smart-UPS SMT, SMX and SRT: text alarms, not F codes

Code lists that attribute numeric F codes to the Smart-UPS SMT and SRT families, including an earlier version of this page, do not match the APC troubleshooting documents cited here: those documents describe faults as messages on the LCD, not numeric F codes. An SMT, SMX or SRT lights its Fault LED or changes its display backlight and writes the condition in words on the LCD. When a Smart-UPS reports an internal fault, APC asks you to note the exact message on the LCD and contact support. Before switching off a unit that is carrying critical load, move that load to another source.

If a Smart-UPS is running on battery while it is plugged in, APC points first at a tripped input breaker or at input voltage that is too high, too low or distorted, and suggests testing the outlet and trying another circuit.

7. Back-UPS ES: reading the LED patterns

The compact Back-UPS ES units have no display. The power button changes colour and flash rate, and a separate indicator lights for building wiring faults. The patterns below are APC’s own definitions for the BE600M1 and BN650M1 family; other Back-UPS ES models use similar but not identical patterns, so check the FAQ for your exact model.

8. What the tone adds to the diagnosis

The beeps confirm the operating state but rarely identify a fault on their own. Schneider Electric publishes the same core patterns for Back-UPS and Smart-UPS. Use them with the display or LEDs, not instead of them.

The full audible guide, with the triage sequence and the difference between the four patterns in the first minute, is on our page explaining why an APC battery backup keeps beeping.

9. Site wiring faults and Canadian power conditions

A site wiring or building wiring fault is not a UPS failure. It means the UPS has detected a problem with the receptacle it is plugged into. Schneider Electric’s FAQ on the indicator lists the three conditions it detects on 120 V Back-UPS and SurgeArrest products: an overloaded neutral (more than 5 V AC measured between neutral and ground), reversed polarity (hot and neutral swapped), and a missing ground, including the use of a three-prong to two-prong adapter. APC’s instruction is to have a qualified electrician inspect the wiring and to use the UPS only on a properly grounded outlet where the indicator stays off; for the Back-UPS ES family it states plainly not to operate the UPS while the indicator is lit.

A second Canadian reality produces many “on battery while plugged in” calls. The reference for supply voltage in Canada is CSA CAN3-C235-83, which sets the normal and extreme variation limits at the service entrance. A line-interactive UPS corrects moderate deviations with its automatic voltage regulator and transfers to battery beyond its configured transfer points; APC’s Back-UPS documents list input voltage out of range as a cause of running on battery while plugged in.

Two practical consequences follow. A UPS that switches to battery repeatedly at a site whose measured voltage sits inside the normal band is more likely set to high sensitivity, or on a circuit with a local problem, than suffering a utility fault; APC’s Back-UPS Pro troubleshooting advice is to adjust the transfer voltage and sensitivity range, and its Smart-UPS advice is to test the outlet and try another circuit. On generator power, which is common at remote and industrial sites across Canada, APC notes that frequency deviations beyond about ±5% put a Back-UPS on battery, and that a generator should be three to five times the size of the total load to hold frequency steady.

10. Symmetra, Galaxy and the safety limits of self-diagnosis

Modular and three-phase systems (Symmetra LX and PX, Galaxy 3500, Galaxy VS, VM and VX) report alarms as text on their own displays and through the network management card event log, and their meaning depends on the module and firmware. We do not publish a code table for them here: the troubleshooting FAQs this guide relies on do not cover them, the alarm lists sit in each system’s own manuals, and the first action for any alarm on these systems is the same: a qualified technician with the event log. For a Symmetra or Galaxy in alarm, call our APC service team rather than acting on a generic list.

Call a technician whenever any of the following is true:

• A Back-UPS Pro or RS shows F03 to F09, or F02 persists with nothing connected.
• A Smart-UPS lights its Fault LED or shows a red backlight with an internal fault message.
• Any Symmetra or Galaxy is in alarm, or any UPS has transferred to bypass on its own and will not return.
• The UPS protects equipment in a hospital, data centre, telecom room or other regulated environment, whatever the code.
• A building wiring fault is shown: that call goes to a licensed electrician first.

For battery-related alarms on critical systems, our UPS battery replacement service handles the replacement, the runtime check and the disposal record, and our preventive maintenance programs catch end-of-life alarms before they turn into dropped loads.

11. Decision checklist and records to keep

Use this sequence the moment an APC UPS alarms:

1. Is the load still powered? If not, and the load is critical, move it to another source before diagnosing.
2. Read the model prefix and what the unit shows (Figure 1). Pick the matching table.
3. Is it a building wiring fault? Stop and call an electrician.
4. Is it F01, F02 or an overload message? Isolate the connected equipment (Figure 2).
5. Is it a battery alarm (L.01, L.02, Replace Battery)? Recharge, self-test, then arrange the replacement. Check the battery’s age against the expected UPS battery service life, and find the right cartridge in the APC RBC cross-reference.
6. Is it anything else (F03 to F09, internal fault, red backlight)? Take the unit out of service and call support or a technician.

Keep a short record for every alarm; a regulated site will be asked for it:

• Date and time, model and serial number, and the exact code or message as displayed (a phone video of start-up is what APC asks for).
• What was connected to the battery outlets and what was disconnected to clear the alarm.
• Battery installation date, last self-test result and any runtime test result.
• Measured receptacle voltage if the unit was running on battery while plugged in, and the sensitivity setting in use.
• Who was called, what was replaced, and the date the unit returned to service.

Other manufacturers use different conventions; if the site also runs Eaton equipment, the equivalent reference is our Eaton UPS alarm and fault code guide.

12. Sources

All Schneider Electric documents were read on 22 September 2026. Where two documents word a code differently, both wordings are shown in the tables. APC, Back-UPS, Smart-UPS, Symmetra and Galaxy are trademarks of Schneider Electric. GDF Technologies is an independent UPS service company and is not affiliated with or endorsed by Schneider Electric.

1. Schneider Electric, FA279106, basic troubleshooting steps for the Back-UPS RS BR1300G / BR1500G (system fault codes F01 to F09; APCRBC124).
2. Schneider Electric, FAQ000264323, basic troubleshooting steps for the Back-UPS Pro BR/BN MS, MS2, M2 and M2-CA models (F01, F02 actions; F03 to F09 not user-correctable).
3. Schneider Electric, FA310801, basic troubleshooting steps for the Back-UPS Pro BX850M to BX1500M.
4. Schneider Electric, FAQ000273815, F02 or F04 error code when turning on Back-UPS (battery connection at power-on).
5. Schneider Electric, FAQ000260345, SMC L.01 or L.02 battery end of life alarm.
6. Schneider Electric, FA279118, basic troubleshooting steps for the Smart-UPS SMC1000 / SMC1500.
7. Schneider Electric, FA299165, basic troubleshooting steps for the Smart-UPS SMT2000 / SMT3000 and RM2U models.
8. Schneider Electric, FA279109, basic troubleshooting steps for the Smart-UPS SMX1500RM2U.
9. Schneider Electric, FA279125, basic troubleshooting steps for the Smart-UPS SRT3KXLT / SRT3KRMXLT.
10. Schneider Electric, FA310519, basic troubleshooting steps for the Back-UPS ES BE600M1 and BN650M1 family.
11. Schneider Electric, FA158817, what the Site Wiring Fault indicator represents.
12. Schneider Electric, FA158827, why an APC Back-UPS might be beeping (alarm patterns, generator frequency note).
13. Schneider Electric, FA156554, descriptions of Smart-UPS audible alarms.
14. CSA Group, CAN3-C235-83, Preferred voltage levels for AC systems, 0 to 50 000 V, Table 3, as reproduced in Newmarket-Tay Power Distribution, Conditions of Service COS-230-05, Voltage Guidelines.