370 on a Fujitsu WATERSTAGE split: Thermodynamic source / outdoor unit fault

What it means

The manual says: Thermodynamic source / outdoor unit fault.

Part involved: sensor/controller: Outdoor unit.

Likely causes:

• A fault in the outdoor unit (read LED code on hydraulic interface board); may show for a few tens of seconds after power-up while outdoor unit self-tests.

Is it safe?

The heat pump stops until the fault clears or is reset.

Resetting itself: no.

Manual reset: no.

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. After a power cut this code can show for a few tens of seconds while the outdoor unit self-tests - wait a few minutes before calling an engineer. Check the outdoor unit is not blocked by leaves, snow or ice (look only). 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

• If it persists read the LED code on the hydraulic interface board and outdoor PCB (see codes 11-A9/AC).

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

• Fault symbol on user interface; press Info key to read error; heat pump operation despite error: No

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.5, p.8; CMD p.7 · Reviewed 23 September 2026

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026