F.629 on a Viessmann One Base: Lead break, return temperature sensor, heating/ cooling circuit 4

What it means

The manual says: Lead break, return temperature sensor, heating/ cooling circuit 4.

Part involved: Lead break, return temperature sensor, heating/ cooling circuit 4.

When it shows: System characteristics: Mixer closes. Heating circuit pump switches off.

Likely causes:

• Lead break, return temperature sensor, heating/ cooling circuit 4.

Is it safe?

The manual does not say whether the heat pump stops.

Resetting itself: Not stated per code in Viessmann document 6171807 GB 10/2021.

Manual reset: User may acknowledge via HMI after noting the code (6249162 p.57); the message returns if the cause persists.

What you can check

• On the HMI, tap the message and "?" to read the system characteristics and Viessmann's user tips; note the code (e.g. F.xxx) and text for your contractor (operating instructions 6249162 p.57).
• Acknowledge the message once via the HMI; do not acknowledge fault messages several times in quick succession (6249162 p.58) - notify your contractor.
• Check the operating program, time program and room temperature settings on the HMI.

What the engineer will check

• Check the settings in the commissioning assistant. Adjust settings if required. Check resistance value.
• (NTC 10 k) at mixer extension Ω kit. Replace sensor if required.

Who to ask for: Viessmann-trained heat pump installer/service engineer.

What you might notice

• Cooling mode confusion / blowing cold
• Some rooms cold

Sources

• Installation and service instructions Vitocal 150-A, AWO(-M)-E-AC/AWO(-M)-E-AC-AF 151.A (6171807, 2021-10) ↗ · 6171807 VSA00001 · page 107 · Reviewed 23 September 2026

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

Reviewed: 23 September 2026