A9 on a Viessmann Vitotronic 200: Current phase L1 of compressor too high

What it means

The manuals for this range word it in different ways:

• Current phase L1 of compressor too high.
• Vitocal 300-G/333-G: Current phase L1 of compressor too high.
• Heat pump: Compressor is switched off.; Central heating and DHW heating occurs with other heat sources enabled for this purpose, e.g. an external heat generator or instantaneous heating water heater. ….

Part involved: Current phase L1 of compressor too high; Vitocal 300-G/333-G: Current phase L1 of compressor too high; Heat pump.

When it shows: Type "Note" (H) - triggers "07 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Compressor OFF Compressor is switched off.; Central heating and DHW heating occurs with other heat sources enabled for this purpose, e.g. an external heat generator or instantaneous heating water heater.; The operation with booster heaters depends on the setting "Operating status after ….

Likely causes:

• Current phase L1 of compressor too high.
• Vitocal 300-G/333-G: Current phase L1 of compressor too high.
• Heat pump faulty.
• Safety high pressure switch has responded: See "C9 Refrigerant circ (SHD)".
• One of the following events has occurred:.
• High pressure limit was exceeded 3 times within 60 min (Vitocal 100-S/111-S only).
• Low pressure limit was undershot twice within 40 min (Vitocal 100-S/111-S only).
• Flow switch has responded 9 times.

Is it safe?

The heat pump stops until the fault clears or is reset.

Resetting itself: Not stated in 6150133.

Manual reset: Refrigerant circuit messages cannot be acknowledged in the message history (6150133 p.155); acknowledge the resulting 05/07/A9 message on the control unit.

What you can check

• This code is only visible to engineers in the service menu (Diagnosis > Refrigerant circuit > Message history); homeowners see message 05, 07 or A9 - note that code and call your contractor.
• Note the 2-character message code and message text shown on the Vitotronic 200 display (press OK for details) and give them to your heating contractor (operating instructions 5817913 p.56).
• You may acknowledge the message once with OK as described in the Vitotronic 200 operating instructions; if it reappears, do not keep acknowledging it - call your contractor.
• Check the heat pump breaker/isolator in the consumer unit is ON (do not repeatedly reset a breaker or RCD that keeps tripping).
• Check the heating system pressure gauge reading and report a low reading to your contractor; make sure the filling loop is closed.
• Check the outdoor unit (air source models) is not blocked by leaves, debris or snow.

What the engineer will check

• Test coil resistance at compressor. Resistance on all windings must be similar (±10 %). For mΩ use a suitable measuring device.
• Replace compressor and/or inverter if required.
• Replace compressor and/or inverter if necessary.
• Calling up further heat pump messages: "Message history".
• Observe messages from the refrigerant circuit controller: "Diagnosis" "Refrigerant circuit" "Message history".
• Check flow rates.
• Check safety high pressure switch.
• Check flow and return temperature sensors in the primary/secondary circuit. Note After removing the fault, switch the appliance first off and then on again.

Who to ask for: Viessmann-trained heat pump engineer with F-gas refrigeration certification.

Certification: F-gas certificate (UK F-gas Regulations; Viessmann: refrigerant work by authorised refrigeration engineers only).

What you might notice

• No heating
• No hot water
• Keeps stopping / cutting out
• Heat pump will not start
• Unexpected backup / auxiliary heater use

Sources

• Service instructions Vitotronic 200 type WO1C heat pump control unit (air/water split & monobloc, brine/water) (6150133, 2023-07) ↗ · 6150133 VSA00004 · pages 171, 178, 100 · Reviewed 23 September 2026

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

Reviewed: 23 September 2026