31 on a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump: Sensor error: BT63 flow sensor: No contact with the sensor (external supply line after heater)
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 31 means: Sensor error: BT63 flow sensor: No contact with the sensor (external supply line after heater). The heat pump keeps running with some functions stopped. 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: Sensor error: BT63 flow sensor: No contact with the sensor (external supply line after heater).
Part involved: Sensor error: BT63 flow sensor.
When it shows: No contact with the sensor (external supply line after heater).
Likely causes:
- Open circuit or short circuit on sensor input; defective sensor.
On the Hydrolution HM gen 1 (HMA100V/HMS140V integrated-controller systems)
The manual says: Sensor fault, high pressure (BP4).
Part involved: S. fault HP.
Likely causes:
- Open-circuit or short-circuit on sensor input; sensor does not work.
Is it safe?
On the Hydrolution (split, FDCW outdoor units)
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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? | 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 (switch SF1 off/on per user manual).
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. The user manual allows resetting by switching the indoor unit off and on with switch SF1; if the alarm cannot be reset, hold the operating-mode button to select 'Add. heat only' (immersion heating) and contact the installer. Recurring alarms mean an installation fault.
What the engineer will check
On the Hydrolution (split, FDCW outdoor units)
- Per manual 'May be due to': Open circuit or short circuit on sensor input; defective sensor. Check sensor wiring/resistance, settings, flow (particle filter, pump) and supply as applicable.
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)
- Indoor-unit checks per HM-T-193 flowchart: check sensors BT12/BT15/BT3, water-side service valves, valves QM31/QN11, strainer, air bleed, circulation pump setting (menu 2.2.5) and relay board AA6; check refrigerant charge/leaks for defrost alarms.
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.78 (printed 76) · 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.156; flowchart p.168 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026