42 on a Hitachi heat pump: Pressure ratio increase abnormality
Short answer
On a Hitachi heat pump, 42 means: Pressure ratio increase abnormality. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the Yutaki M (RASM monobloc)
The manual says: Pressure ratio increase abnormality.
Part involved: compression ratio.
Likely causes:
- Compression ratio above limit (Yutaki M R32 4-7 HP).
On the Yutaki S (split air-to-water)
The manual says: Heating overload (outdoor).
Likely causes:
- Outdoor heat exchanger clogging/short circuit; expansion valve closed-locked.
Is it safe?
On the Yutaki M (RASM monobloc)
| 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.
No need to switch off — call an engineer if it comes back.
Resetting itself: no.
Manual reset: Stop the unit (via unit controller / power off-on) after the cause is fixed.
On the Yutaki S (split air-to-water)
| 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.
No need to switch off — call an engineer if it comes back.
Resetting itself: unknown.
Manual reset: unknown.
What you can check
- Read and note the alarm code and any retry code (e.g. P70) shown on the unit controller.
- Check the heat pump supply breaker/isolator at the consumer unit is ON.
- Restart once via the unit controller (or switch off/on at the isolator) - the manual states the unit must be stopped to release most alarms; if the code returns, call an engineer.
What the engineer will check
- Follow SMGB0146 section 6.7 troubleshooting flowchart for alarm 42.
- Check refrigerant charge and for leaks (F-gas qualified).
- Check expansion valve operation and strainers for clogging.
- Check heat exchanger/fan for blockage, pressure and temperature sensors.
- Check water flow (low flow causes high-pressure trips in heating).
Who to ask for: F-gas certified refrigeration / heat pump engineer (Hitachi Yutaki trained).
Certification: F-gas certificate (Category I); A2L (R32) flammable refrigerant training for R32 units.
What you might notice
Sources
- Service Manual - YUTAKI R32 / R410A series (Split, Monobloc and Hydrosplit systems) ↗ · SMGB0146 rev.0 - 12/2022 · page table printed p.654 (viewer p.680); troubleshooting flowchart printed p.703 (viewer p.729) · Reviewed 23 September 2026
- YUTAKI S Series Technical Catalogue (RWM-(H)FSN3E indoor units; RAS-H(V)RNME-AF / RAS-HVRN2 outdoor units) ↗ · TCGB0070 rev.3 - 04/2012 · page p.163 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026