426 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution: Temp. safety stop inv.: A temporary internal fault in the inverter has occurred
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 426 means: Temp. safety stop inv.: A temporary internal fault in the inverter has occurred. 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: Temp. safety stop inv.: A temporary internal fault in the inverter has occurred.
Part involved: Temp. safety stop inv.
When it shows: A temporary internal fault in the inverter has occurred.
Likely causes:
- Disruption in supply voltage.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes.
Manual reset: no.
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). This is an information message; it resets automatically when the cause is removed.
What the engineer will check
- Per manual 'May be due to': Disruption in supply voltage. 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.
What you might notice
Sources
- Installation Manual - Air to Water Heat Pump using R32 refrigerant, Hydrolution (HM) ↗ · PSC012D169 (202201) · page PDF p.89 (printed 87) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026