84 on a Fujitsu heat pump: Current sensor error
Short answer
On a Fujitsu heat pump, 84 means: Current sensor error. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the AIRSTAGE Monobloc Comfort (R32)
The manual says: Current sensor error.
Part involved: Current sensor error.
On the WATERSTAGE split (Comfort, High Power, Super High Power)
The manual says: BSB, address collision.
Part involved: sensor/controller: BSB bus.
Likely causes:
- Two devices with the same bus address.
Is it safe?
On the AIRSTAGE Monobloc Comfort (R32)
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: unknown.
Manual reset: unknown.
On the WATERSTAGE split (Comfort, High Power, Super High Power)
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: no.
Manual reset: no.
What you can check
On the AIRSTAGE Monobloc Comfort (R32)
- Note the error number shown after pressing the user interface. Check the heat pump breakers/isolators are on. Check the outdoor unit is not blocked by leaves, snow or ice (look only).
On the WATERSTAGE split (Comfort, High Power, Super High Power)
- Note the error number shown after pressing the Info (i) key on the Waterstage user interface. Check the heat pump breakers/isolators are on. The user manual RESET key/'Reset HP' is for installers ('do not use during normal operation') - contact your installer if the fault remains.
What the engineer will check
On the AIRSTAGE Monobloc Comfort (R32)
- Engineer diagnosis of outdoor monobloc unit (sensors, PCB/IPM, fan, compressor, refrigerant circuit - F-gas; water flow/circulator for AE/9B).
Who to ask for: Heat pump / refrigeration engineer (manufacturer-trained).
Certification: F-gas certification for refrigerant-circuit work; electrical competence (Part P/18th Edition) for PCB/wiring.
On the WATERSTAGE split (Comfort, High Power, Super High Power)
- Check addressing of room units/controllers.
Who to ask for: Heat pump / refrigeration engineer (manufacturer-trained).
Certification: F-gas certification for refrigerant-circuit work; electrical competence (Part P/18th Edition) for PCB/wiring.
What you might notice
Sources
- INSTALLATION - General Monobloc Comfort Duo, Monobloc 2 services air/water heat pump ↗ · U0780597_2690_EN_3 · page p.37 · Reviewed 23 September 2026
- AIR TO WATER Comfort series DESIGN & TECHNICAL MANUAL Split type ↗ · DA_CF002EF_07 · page DTM p.110 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026