Engineering guide
Mitsubishi 9900D fault codes explained
On a Mitsubishi 9900D, a code with A as its second letter is an alarm and a code with F is a failure, according to section 7.0 of the owner's manual U-ENM00055. A code starting with U comes from the bypass module and one starting with M from a UPS module. A Major failure moves the load to the static bypass line; with a Minor failure the UPS keeps running, but the cause still has to be found. The tables below give the manual's meaning and guidance for the codes operators see most.
Key facts
- UA807 INVERTER OVERLOAD: load current between 105% and 149% of rated current
- Major failure: load transferred from the inverter to the static bypass line
- UA805 OVERLOAD: output load current above 100% of rated current
- UA834 BATTERY DEPLETED: battery voltage at the depleted level (400V)
How to read a 9900D code
The first letter names the part of the system that raised the code: U for the bypass module, M for a UPS module. The second letter separates alarms (A) from failures (F). Next to the code, the display shows a status message and, for most codes, a guidance line such as DECREASE LOAD or CALL SERVICE ENGINEER. The audible alarm is intermittent or continuous depending on the code.
Before anything is reset, write down the code, the message and the time. The manual asks for that record first, and it is what a technician needs to read the event log.
Overload codes
| Code | Message on the display | What the manual says | What to do |
|---|---|---|---|
| UA805 | OVERLOAD | Output load current above 100% of rated current. | Reduce the load (DECREASE LOAD). |
| UA806 | INVERTER OVERLOAD | Peak load current above 220% of rated current. | Reduce the load. |
| UA807 | INVERTER OVERLOAD | Load current between 105% and 149% of rated current. | Reduce the load. |
| UA808 | OVERLOAD | Load current above 100% of rated current while on bypass supply. | Reduce the load. |
| UA810 | OVERLOAD | Short-time overcurrent above 150% of rated current. | Reduce the load. |
| UA864 | MODULE OVERLOAD | A UPS module is overloaded. | Reduce the load. |
| UF258 | LOAD ABNORMAL | Load transferred for overload more than 4 times within 5 minutes. | Call a technician (CALL SERVICE ENGINEER). |
Most overloads follow a change on the protected side: a new rack, a cooling unit moved onto the UPS, or a second feed that dropped and pushed its share onto this one. Shedding non-critical load is the right first move. UF258 is different: the UPS has already transferred four times in five minutes, so the load should stay where the UPS put it until a technician has looked.
Input and bypass codes
| Code | Message on the display | What the manual says | What to do |
|---|---|---|---|
| UA812 | BYPS.VOLT.OUT RNG. | Bypass voltage out of range (detection below 87% or above 112%). | Check the bypass supply (CHECK BYPASS INPUT). |
| UA813 | BYPS.PHASE ABNL. | Bypass conductors not in the proper phase sequence. | Have an electrician check the bypass wiring. |
| UA814 | BYPS.FREQ.OUT RNG. | Bypass frequency out of range. | Check the bypass supply; a generator may be running. |
| UA815 | TRANSFER PROHIBITION | Transfer to bypass unavailable because the bypass is abnormal. | Avoid any planned transfer until the bypass is back. |
| UA863 | MOD.IN.VOLT. | Input voltage abnormal on a module. | Check the input power source (CHECK INPUT POWER SOURCE). |
These codes describe the supply, not the UPS. Check whether the utility had an event, whether the generator is running, and whether electrical work was done upstream. A phase sequence error after such work is a wiring correction for an electrician.
Battery codes
| Code | Message on the display | What the manual says | What to do |
|---|---|---|---|
| UA824 | CB2 OPEN | The battery circuit breaker (CB2) is open. | Confirm whether it was opened for work; the manual's guidance is TURN ON CB2. |
| UA834 | BATTERY DEPLETED | Battery voltage has reached the depleted level (400V). | Call a technician. |
| UA837 | BATTERY DEPLETED2 | Calculated battery temperature at its limit after a long, heavy discharge. | Call a technician. |
| UA866 | BATT.END WA | Approaching end of battery. | Prepare the loads for shutdown if the outage continues. |
| UA867 | OVERTEMPERATURE | Battery overtemperature. | Call a technician; check the battery room cooling. |
| UF157 | BATTERY OVERTEMP. | Overtemperature detected at the batteries. | Check the batteries' room temperature (CHECK BATTERY). |
| UF158 | BATTERY LIQUID LOW | Low electrolyte level (wet cell batteries). | Have the battery string inspected (CHECK BATTERY). |
| UF162 | BATTERY ABNORMAL | Failure found by the battery self-check. | Call a technician. |
A breaker left open after maintenance is the most common cause of UA824; until CB2 is closed by someone trained on the procedure, the UPS has no battery backup.
Switches, emergency stop and external alarms
| Code | Message on the display | What the manual says | What to do |
|---|---|---|---|
| UA817 | EMERG.STOP ACTIV. | Emergency stop activated by the EPO switch or an external contact. | Find out who or what triggered the EPO before any restart. |
| UA830 | INTERLOCK SWITCH ON | The interlock switch was turned on. | Check whether maintenance is under way. |
| UA831 | EMERG.BYPS.SW.ON | The emergency bypass switch has been turned on. | Call a technician. |
| UA833 | 52L OPEN | The load circuit breaker (52L) is turned off. | Confirm whether it was opened on purpose. |
| UA890 | EXTERNAL ALARM | An external alarm relay turned on. | Check the equipment wired to that input. |
Codes that need a technician
| Code | Message on the display | What the manual says | What to do |
|---|---|---|---|
| UF202 | INVERTER UV. | Output voltage dropped (Major). | Call a technician. |
| UF301 | CTRL.CIRCUIT ERR. | AD (analog-to-digital) reference abnormal (Major). | Call a technician. |
| UF340 | MODULE MAJOR FAULT | Module major fault. | Call a technician. |
| UF401 | 52S ABNORMAL | Contactor 52S failed to close (Major). | Call a technician. |
| UF453 | STS TEMP. ABNOMAL | Static switch thyristor temperature abnormal. | Call a technician. |
| MF103 | DC UNDERVOLTAGE | DC voltage below the undervoltage level, under 630Vdc (Major). | Call a technician. |
| MF119 | DC GROUND FAULT | DC ground fault detected (Major). | Call a technician. |
| MF201 | INVERTER OV. | Output overvoltage detected (Major). | Call a technician. |
Every failure code with CALL SERVICE ENGINEER as its guidance, and every Major code, is a technician job. Leave the cabinet closed, leave the load on the path the UPS chose, and keep the record of codes. Our Mitsubishi UPS technicians read the 9900D event log on site and plan the repair from it; if a module has to be pulled and tested, our repair bench takes it from there.
Related pages
Common questions
What is the difference between UA and UF codes on a 9900D?
In the owner's manual, the second letter A marks an alarm and F marks a failure. U means the bypass module raised it; M means a UPS module did.
What does UA805 OVERLOAD mean?
The output load current has exceeded 100% of the rated current. The guidance is to decrease the load.
What does UA824 CB2 OPEN mean?
The battery circuit breaker CB2 is open, so there is no battery backup. Check whether it was opened for maintenance.
What is a Major failure?
The manual defines it as a failure where the load is transferred from the inverter to the static bypass line.
Can I clear a UF code myself?
Record it, then call a technician. Most UF and MF codes carry the guidance CALL SERVICE ENGINEER.
Sources, checked October 4, 2026:
- Mitsubishi Electric 9900D Series UPS Owners / Technical Manual U-ENM00055, Rev 7 (08/25/2025), section 7.0, p. 7-1
- Mitsubishi Electric 9900D Series UPS Owners / Technical Manual U-ENM00055, Rev 7 (08/25/2025), Table 7.2 notes, p. 7-10
- Mitsubishi Electric 9900D Series UPS Owners / Technical Manual U-ENM00055, Rev 7 (08/25/2025), Table 7.1, pp. 7-2 to 7-3, 7-2 to 7-4
- Mitsubishi Electric 9900D Series UPS Owners / Technical Manual U-ENM00055, Rev 7 (08/25/2025), Tables 7.1 and 7.2, p. 7-4 to 7-8
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