Viessmann Error Code F.668: DHW reheating sensor TS4 open circuit (Boiler)
Viessmann error code F.668 on a boiler means: DHW reheating sensor TS4 open circuit. It is also called Viessmann diagnostic code F.668.
Homeowner? What Viessmann code F.668 means for you
Usually minor Your boiler is reporting: DHW reheating sensor TS4 open circuit. This code is usually informational or minor. The safe checks below often clear it; call us if it keeps showing up.
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.668 mean?
No solar preheating function.
Applies to: Vitodens 200-W B2HE 85, 120, 150, 199
Common causes of Viessmann code F.668
- Open circuit TS4
Viessmann models that use code F.668
B2HE-85B2HE-120B2HE-150B2HE-199
Other Viessmann Boiler codes
All Viessmann Boiler error codes →
Viessmann code F.668: homeowner questions
What does Viessmann code F.668 mean?
DHW reheating sensor TS4 open circuit. No solar preheating function.
Is it safe to keep using my boiler with code F.668?
Usually yes. Code F.668 is normally minor, but if it keeps coming back have it checked.
Can I reset Viessmann code F.668 myself?
You can try one reset (power off for 30 seconds, then on). If Viessmann code F.668 comes back, the cause is still there and a licensed technician should check it.
Who can fix Viessmann code F.668 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.
Homeowner help: Boiler Not Heating: What to Check
Troubleshooting guides
- Condensing/corrosion of conventional boiler All brands · Boiler
- No heat or some zones cold All brands · Boiler
- Noise All brands · Boiler
- Poor heat / circulation All brands · Boiler
- Relief valve weeps every cycle / pressure climbs All brands · Boiler
- Short cycling / wasted fuel All brands · Boiler
- Symptoms of air in system All brands · Boiler
- Uneven heating All brands · Boiler
- Won't fire All brands · Boiler
- Acknowledged fault comes back Viessmann · Boiler
- Actuator test: drive fan, burner, pump and 3-way valve by hand Viessmann · Boiler
- Clear the service message (P.1 / P.8) Viessmann · Boiler
- Condensate not draining, burner shuts down Viessmann · Boiler
- Filling and purging programs (20 min auto-stop) Viessmann · Boiler
- Lag boilers lose contact with lead boiler (cascade) Viessmann · Boiler
- Open the Service menu (password viservice) Viessmann · Boiler
- Where the fault history lives Viessmann · Boiler
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.