OU-A5 on a Fujitsu WATERSTAGE split: Low pressure abnormal
Short answer
On a Fujitsu heat pump, OU-A5 means: Low pressure abnormal. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Low pressure abnormal.
Part involved: outdoor unit PCB/refrigerant circuit.
Likely causes:
- Connector connection failure; pressure sensor failure; main PCB failure; gas leak / low charge.
Is it safe?
Switch the heat pump off at its isolator and call an engineer.
| 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.
What you can check
- 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 hydraulic unit shows error 370; the specific code is shown by LED flashes on the hydraulic interface PCB / outdoor PCB, which only an engineer should access. Check the outdoor unit is not blocked by leaves, snow or ice (look only). If the unit repeatedly locks out, leave it off and call an engineer (possible refrigerant loss).
What the engineer will check
- Per SHP maintenance manual 'Clear' procedure: Connector connection failure; pressure sensor failure; main PCB failure; gas leak / low charge. Access outdoor PCB (ERROR LED, press ENTER to read flash count), check connectors, sensor resistances, supply voltage (198-264 V AC 1ph), serial signal (AC 70-130 V between terminals), valves/EEV, refrigerant charge (F-gas), fan motor, compressor windings, IPM/PFC boards.
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
- Maintenance Document - Air to Water Heat Pump Split single service and integrated DHW type (Super High Power) ↗ · 2105_EN_1 · page SHP SM p.6, p.29 (Clear 26); DTM p.62 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026