04 on a Hitachi Yutaki M: Abnormal transmission between inverter PCB and main PCB
Short answer
On a Hitachi heat pump, 04 means: Abnormal transmission between inverter PCB and main PCB. 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: Abnormal transmission between inverter PCB and main PCB.
Is it safe?
| 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).
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 04.
- Isolate power and wait for capacitor discharge before opening the electrical box.
- Check inverter PCB, IPM/DIP-IPM, current sensor wiring and connectors.
- Check compressor and fan motor windings; check heat exchanger for clogging.
Who to ask for: Heat pump service engineer (electrical/inverter diagnostics).
Certification: Competent electrical worker (BS 7671); isolate and wait 10 min for capacitor discharge.
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
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026