73 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution HM gen 1: Outdoor temperature below 0 C and operating mode does not permit heating (resets above 1 C)
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 73 means: Outdoor temperature below 0 C and operating mode does not permit heating (resets above 1 C). 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: Outdoor temperature below 0 C and operating mode does not permit heating (resets above 1 C).
Part involved: Freeze prot.
When it shows: Outdoor temperature below 0 C and operating mode does not permit heating (resets above 1 C).
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
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. No action needed if it clears; if it keeps recurring, contact the installer.
What the engineer will check
- Review flow rate, set temperatures and defrost conditions; if escalated see alarms 5/6/7/14/16.
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.155 (printed 154) - 'Alarm with automatic reset' · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026