E42 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution HM gen 1: Voltage from the inverter outside the parameters four times within 30 minutes (troubleshooting chart titled 'E42 - Current cut')
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, E42 means: Voltage from the inverter outside the parameters four times within 30 minutes (troubleshooting chart titled 'E42 - Current cut'). 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: Voltage from the inverter outside the parameters four times within 30 minutes (troubleshooting chart titled 'E42 - Current cut').
Part involved: Inverter error.
When it shows: 4x/30 min.
Likely causes:
- Incoming power supply interference; service valve closed; insufficient refrigerant; compressor fault; defective inverter PCB in outdoor unit.
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. Check the outdoor unit is not blocked by leaves, snow or ice and its drain is clear (look only - do not open the casing). 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
- 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.157; flowchart p.176 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026