251 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution: Com. err. MODBUS: Communication with the accessory (MODBUS40M) has been missing for 15 seconds
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 251 means: Com. err. MODBUS: Communication with the accessory (MODBUS40M) has been missing for 15 seconds. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Com. err. MODBUS: Communication with the accessory (MODBUS40M) has been missing for 15 seconds.
Part involved: Com. err. MODBUS.
When it shows: Communication with the accessory (MODBUS40M) has been missing for 15 seconds.
Likely causes:
- Check the communication.
Is it safe?
| 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).
What you can check
- 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.
What the engineer will check
- Per manual 'May be due to': Check the communication. 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.
What you might notice
Sources
- Installation Manual - Air to Water Heat Pump using R32 refrigerant, Hydrolution (HM) ↗ · PSC012D169 (202201) · page PDF p.81 (printed 79) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026