P1 on a Daikin Altherma HT: High voltage of capacitor in main inverter circuit

What it means

The manual says: High voltage of capacitor in main inverter circuit.

Part involved: power supply / inverter electrical.

When it shows: When the aforementioned voltage waveform becomes identical with the waveform of the power supply open phase.

Likely causes:

• Defect of main circuit capacitor; Improper main circuit wiring; Defect of outdoor unit PC board (A2P); Imbalance of phase-to-phase voltages; Open phase; Defect of indoor unit inverter PC board [A4P (1ø) or A5P (3ø)].

Is it safe?

The manual does not say whether the heat pump stops.

Resetting itself: unknown.

What you can check

• Read the full error code shown on the user interface and note it, with the date/time, for your installer.
• Check the heat pump isolator switch(es) and the consumer-unit breaker/RCD for the heat pump are ON. If a breaker or RCD has tripped, do not keep resetting it - leave it and call an engineer.

What the engineer will check

• Follow the service-manual troubleshooting flow chart for this code: check defect of main circuit capacitor; check improper main circuit wiring; check defect of outdoor unit PC board (A2P); check imbalance of phase-to-phase voltages; check open phase; check defect of indoor unit inverter PC board [A4P (1ø) or A5P (3ø)].

Who to ask for: Heat pump engineer / qualified electrician.

Certification: Electrical competence to BS 7671 (Part P where applicable) for mains supply work.

What you might notice

• Heat pump will not start
• Keeps stopping / cutting out
• No heating
• Tripping RCD or breaker

Sources

• Service Manual Daikin ALTHERMA HT ERSQ/ERRQ 011-016 AA V1/Y1 + EKHBRD (ESIE09-08C) ↗ · ESIE09-08C · page 205 · Reviewed 23 September 2026

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

Reviewed: 23 September 2026