Viessmann Error Code F.548: HC4 mixed supply sensor fault (open circuit) (Boiler)
Viessmann error code F.548 on a boiler means: HC4 mixed supply sensor fault (open circuit). It is also called Viessmann diagnostic code F.548.
Homeowner? What Viessmann code F.548 means for you
Check, then call Your boiler is reporting: HC4 mixed supply sensor fault (open circuit). This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.
Safe things you can check yourself
- Check the thermostat: right mode (Heat or Cool), set a few degrees past room temperature, and fresh batteries if it uses them.
- Make sure the unit has power: its on/off switch (often looks like a light switch near the unit) is ON and the breaker in the electrical panel is not tripped.
- Look at the pressure gauge on the boiler. If it reads much lower than usual or near zero, or water is leaking, call a technician.
- Outside, make sure the intake and exhaust pipes or vent cap are not blocked by snow, ice, leaves or a nest.
- Check that the boiler gas valve is open (handle in line with the pipe).
- Try one reset: turn the power off for 30 seconds, then back on. If the code comes back, stop resetting and call a technician.
Call (647) 657-2977Book a service call
Licensed technicians serving Vaughan, Toronto and the GTA.
What does Viessmann error code F.548 mean?
Mixer closes; pump keeps running. (Manual labels both F.548 and F.549 'short circuit'; the F.548 measures also mention commissioning and rotary switch like an open-circuit entry.)
Applies to: Vitodens 200-W B2HE 85, 120, 150, 199
Common causes of Viessmann code F.548
- Sensor fault
- Wrong commissioning setting
Viessmann models that use code F.548
B2HE-85B2HE-120B2HE-150B2HE-199
Other Viessmann Boiler codes
All Viessmann Boiler error codes →
Viessmann code F.548: homeowner questions
What does Viessmann code F.548 mean?
HC4 mixed supply sensor fault (open circuit). Mixer closes; pump keeps running. (Manual labels both F.548 and F.549 'short circuit'; the F.548 measures also mention commissioning and rotary switch like an open-circuit entry.)
Is it safe to keep using my boiler with code F.548?
It depends on the cause. This is often caused by something simple. Try the safe checks below; if the code returns, book a service call before it turns into a no-heat or no-hot-water emergency.
Can I reset Viessmann code F.548 myself?
You can try one reset (power off for 30 seconds, then on). If Viessmann code F.548 comes back, the cause is still there and a licensed technician should check it.
Who can fix Viessmann code F.548 near me?
Aurora Star Tech Inc. repairs Viessmann boilers in Vaughan, Toronto and across the GTA. Call (647) 657-2977 or book a service call online.
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Source: Viessmann Vitodens 200-W B2HE 85/120/150/199 Service Instructions 6167 813-24 (09/2026), Fault messages, p.62-73. Written in our own words as a summary; always check the original manual.
General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.