71 on a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump: Communication fault with PCA base: Communication with PCB (AA2) is missing
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 71 means: Communication fault with PCA base: Communication with PCB (AA2) is missing. 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 Hydrolution (split, FDCW outdoor units)
The manual says: Communication fault with PCA base: Communication with PCB (AA2) is missing.
Part involved: Communication fault with PCA base.
When it shows: Communication with PCB (AA2) is missing.
Likely causes:
- Defective communication cables.
On the Hydrolution HM gen 1 (HMA100V/HMS140V integrated-controller systems)
The manual says: Condenser supply above 60 C more than 120 s after shifting to heating (auto reset below 51 C).
Part involved: High KF.
When it shows: Condenser supply above 60 C more than 120 s after shifting to heating (auto reset below 51 C).
Is it safe?
On the Hydrolution (split, FDCW outdoor units)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no.
Manual reset: yes (select 'reset alarm' in the alarm menu).
On the Hydrolution HM gen 1 (HMA100V/HMS140V integrated-controller systems)
| 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: yes.
Manual reset: no.
What you can check
On the Hydrolution (split, FDCW outdoor units)
- Note the alarm number/text shown on the display. Check the consumer-unit breakers/isolators for the indoor and outdoor units are on. On the RC-HY controller you may select 'reset alarm' once (installation manual: 'in most cases it is enough'); if the red lamp stays or the alarm returns, select 'aid mode' (if enabled) or 'add. heat only' in menu 4.2 and contact the installer.
On the Hydrolution HM gen 1 (HMA100V/HMS140V integrated-controller systems)
- Note the alarm number/text shown on the display. Check the consumer-unit breakers/isolators for the indoor and outdoor units are on. No action needed if it clears; if it keeps recurring, contact the installer.
What the engineer will check
On the Hydrolution (split, FDCW outdoor units)
- Per manual 'May be due to': Defective communication cables. Check communication cabling, DIP switches and power to accessory/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.
On the Hydrolution HM gen 1 (HMA100V/HMS140V integrated-controller systems)
- Review flow rate, set temperatures and defrost conditions; if escalated see alarms 5/6/7/14/16.
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 Manual - Air to Water Heat Pump using R32 refrigerant, Hydrolution (HM) ↗ · PSC012D169 (202201) · page PDF p.79 (printed 77) · Reviewed 23 September 2026
- Technical Manual HM-T-193 - Air to Water Heat Pump (Hydrolution) ↗ · '13 HM-T-193 (updated December 18, 2014) · page ML p.155 (printed 154) - 'Alarm with automatic reset' · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026