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Heat pumps · Error codes · Hitachi
6Hitachi

6 on a Hitachi heat pump: Excessively low or excessively high voltage for the inverter

Short answer

On a Hitachi heat pump, 6 means: Excessively low or excessively high voltage for the inverter. 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: Excessively low or excessively high voltage for the inverter.

Part involved: inverter supply voltage.

Likely causes:

  • Voltage drop in power supply; incorrect wiring or insufficient capacity of power supply wiring.

Is it safe?

Does the heat pump stop?Yes
SeverityFault — 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 6.
  • Measure supply voltage and phase sequence/imbalance at the terminal board.
  • Check supply cable size and connections per manual.

Who to ask for: Electrician / heat pump service engineer.

Certification: BS 7671 competent electrician for supply issues.

What you might notice

Sources

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026