Heat pumps · Error codes · Hitachi
YCC-21XHitachi · Hitachi, all models
YCC-21X on a Hitachi heat pump: Slave unit alarm - module X in alarm state or cascade controller lost communication with module X
Short answer
On a Hitachi heat pump, YCC-21X means: Slave unit alarm - module X in alarm state or cascade controller lost communication with module X. 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: Slave unit alarm - module X in alarm state or cascade controller lost communication with module X.
Part involved: YUTAKI Cascade Controller.
Likely causes:
- X = module number; check the alarm on that module.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: 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 21X.
- Check H-Link / transmission wiring polarity, continuity and terminal tightness.
- Check PCB fuses and DIP-switch/address settings.
- Check power to the other unit (outdoor/indoor/controller).
Who to ask for: Heat pump installer / service engineer (controls wiring).
What you might notice
Sources
- Installation & Operation Manual - YUTAKI series and YUTAKI Cascade Controller (R410A series / R32 series) ↗ · PMML0510 rev.1 - 10/2019 · page p.148 (PDF p.159) · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026