23 on a Hitachi heat pump: Failure of discharge gas temperature thermistor (open/short)
Short answer
On a Hitachi heat pump, 23 means: Failure of discharge gas temperature thermistor (open/short). 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 (RHUE, 1st generation)
The manual says: Failure of discharge gas temperature thermistor (open/short).
On the Yutaki S80 (high-temperature cascade split)
The manual says: Abnormal operation of the thermistor for the discharge gas (Td) - Yutaki S80 R134a stage.
Part involved: S80 discharge gas thermistor (Td, THM9 at PCB).
Likely causes:
- Loose, disconnected, broken or short-circuited connector.
Is it safe?
On the Yutaki M (RHUE, 1st generation)
| 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: Power OFF/ON to remove alarms in Yutaki M and system controller (TCGB0090 p.141 for F16/F17).
On the Yutaki S80 (high-temperature cascade split)
| 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.
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 23.
- Check thermistor/sensor connector seating and wiring on the PCB.
- Measure thermistor resistance against the characteristic table; replace sensor if out of range.
- Replace PCB if sensor and wiring are good.
Who to ask for: Heat pump service engineer.
Certification: None specific (electrical competence for PCB work).
What you might notice
Sources
- YUTAKI M Series RHUE-3AVHN1 / RHUE-(3-6)A(V)HN-HM Technical Catalogue ↗ · TCGB0090 rev.0 - 12/2013 · page p.139 (PDF p.147) · Reviewed 23 September 2026
- Service Manual - YUTAKI R32 / R410A series (Split, Monobloc and Hydrosplit systems) ↗ · SMGB0146 rev.0 - 12/2022 · page table printed p.653 (viewer p.679); troubleshooting flowchart printed p.692 (viewer p.718) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026