424 on a Mitsubishi Heavy Industries Air-Conditioning Europe Hydrolution: Triggered pressure switch: Temporarily triggered pressure switch
Short answer
On a Mitsubishi Heavy Industries Air-Conditioning Europe heat pump, 424 means: Triggered pressure switch: Temporarily triggered pressure switch. 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: Triggered pressure switch: Temporarily triggered pressure switch.
Part involved: Triggered pressure switch.
When it shows: Temporarily triggered pressure switch.
Likely causes:
- Poor heating medium flow; lack of refrigerant.
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). Check radiator/underfloor thermostats in the house are not all closed (user manual). Check the heating system pressure gauge reads in the normal range (about 0.5-1.5 bar when cold for Hydrolution per user manual). This is an information message; it resets automatically when the cause is removed.
What the engineer will check
- Per manual 'May be due to': Poor heating medium flow; lack of refrigerant. Refrigerant/outdoor-unit work (service valves, charge, sensors Tho-R/A/D/S, LPT, 63H1, fan motor, inverter/control PCBs) must be done by an F-gas certified engineer.
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