Fujitsu Mini-Split Code 77
Soft lockoutShown as: OPERATION lamp blinks 7 times, TIMER lamp blinks 7 times, ECONOMY lamp blinking continuously
Heat sink thermistor error. The heat sink temperature sensor built into the power module is open or shorted.
Manual wording: “Heat Sink Thermistor Error - Heat sink temperature thermistor (built-in IPM) is open or shorted.”
Documented for: RGLX, RL2/RLS3/RLB/RLXB, RLF/RLF1 outdoor units
Likely causes, most common first
- Failed inverter PCB (the sensor is built into the module on wall-mounted models)
- Erroneous connection or removed connector
- Thermistor resistance out of specification
What you can safely check yourself
- Turn the power off at the outdoor disconnect and breaker for about 5 minutes, then restore power
When to call a technician
On wall-mounted models this sensor is part of the inverter board, so the fix is a board replacement by a licensed technician.
A Fujitsu mini-split throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- 410A Mini-Split Troubleshooting Guide (Part # 6-26-FG2020 Rev. 5/2019) (Fujitsu General America, Inc., All R410A Halcyon mini-splits: RGLX, RL2/RLS3/RLS3H/RLS3Y/RLB/RLXB/RLXEH wall mounted, RLF/RLF1 multi-zone indoor units, RLXFZ multi-zone outdoor units, RML/RMLQ, RULX/RSLX universal-mount, RCLX cassettes) · checked 2026-08-25