E5 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution HM gen 1: Communication between the outdoor unit and indoor unit is not made
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, E5 means: Communication between the outdoor unit and indoor unit is not made. 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: Communication between the outdoor unit and indoor unit is not made.
Part involved: OU Com. error.
Likely causes:
- Isolator switch for outdoor unit off; electrical noise from other equipment.
Is it safe?
| 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
- 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 says: check that any circuit breakers to the outdoor unit are not off ('OU power failure / OU Com. error'). Reset via switch SF1 once; if it persists contact the installer.
What the engineer will check
- Refrigerant-side and PCB checks per HM-T-193 flowchart: check sensor resistances (Tho-R/A/D/S, LPT), 63H1 switch, service valves, refrigerant charge, fan motor and fuse F2, inverter/control PCBs, supply voltage.
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
- Technical Manual HM-T-193 - Air to Water Heat Pump (Hydrolution) ↗ · '13 HM-T-193 (updated December 18, 2014) · page ML p.156 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026