Mitsubishi Electric mini split error code Fb
Indoor unit control board fault. Also shown as FB.
- TypeFault
- SystemControl board
- Can I fix it?Technician job
What Fb means on a Mitsubishi Electric
The indoor control board failed its self-check or memory read.
On this platform, Fb is a fault: something has failed or is disconnected and needs diagnosis.
Common causes
- A surge event.
- Water on the board.
- A failed board.
How to fix it yourself
Do the reset and the visual checks, but the underlying repair needs a technician with a meter and the service manual.
- Reset the system: switch the breaker (or the outdoor disconnect) off, wait a full five minutes so the boards fully discharge, then switch it back on and restart from the remote.
- If the code comes straight back after a reset, the fault is real and not a glitch.
Switch off power at the breaker before opening either unit. Capacitors in the outdoor unit hold a charge for several minutes after power is cut.
When to call a technician
Board replacement.
Which Mitsubishi Electric units show Fb
This page applies to Mitsubishi Electric M-Series (MSZ), Mr. Slim (PKA/PCA), Hyper-Heating H2i and MXZ multi-zone units. Mitsubishi Electric's own documentation lists Fb as "Indoor unit control board fault", the same definition used across the Mitsubishi Electric it is built on. If your unit shows a code with a different shape (for example a four-character code, or a plain number), it is from a different table; use the lookup with your exact code.
Where Mitsubishi Electric shows this code
Wall units without a wired controller signal a fault by blinking the operation indicator lamp; the code itself shows on a wired controller (MHK/PAR series) or on the wireless remote after entering its check mode (see your manual). On the outdoor unit, the control board LEDs blink the same code. P codes are indoor-unit faults, E codes are communication faults, F codes are outdoor-unit faults and U codes are outdoor-unit and compressor faults.
Other Mitsubishi Electric codes
Same part of the system as Fb:
Fb on other brands
The same characters do not always mean the same thing. Check the brand on your unit.
- Zone Air Fb Cooling or heating overload limit (frequency reduction protection)
- Trane Fb Cooling or heating overload limit (frequency reduction protection) / Indoor unit control board fault
Questions people ask
Can I reset a Mitsubishi Electric Fb code myself?
Yes. Turn the system off at the breaker for five minutes, then restart it. If Fb comes back, the fault is real and needs the checks above.
Is it safe to keep running a Mitsubishi Electric showing Fb?
The unit will usually refuse to run, or run with reduced output, until the fault is cleared. Repeated resets do not fix anything and can mask a problem that is getting worse.
What will a technician check for Fb?
Board replacement. Ask for the measurements, not just a parts quote.
Sources
- Mitsubishi Electric M-Series and Mr. Slim service manuals (self-diagnosis tables)
Updated September 2026. Codes and definitions vary by model year. If your manual disagrees with this page, your manual wins. Send a correction.