Viessmann F6 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann F6 fault code mean?
The F6 fault code on a Viessmann boiler indicates that the boiler water temperature sensor is producing a reading that falls outside the expected range. On many Viessmann models — particularly those fitted with a dual-element boiler water temperature sensor — the fault triggers when the two elements within that single sensor begin to report temperatures that differ too widely from each other. The boiler's control board detects this discrepancy, locks out the burner, and displays F6 to alert you that it cannot trust the temperature data it is receiving. The result is that the boiler stops firing, leaving you without heating or hot water until the fault is resolved. It is worth noting that F6 can carry slightly different meanings across the Viessmann range: on certain models it relates specifically to the dual-element sensor differential described above, while on a small number of other models it can point to a gas-related error. Always cross-reference with your specific boiler's handbook or Viessmann's official fault-code lookup tool to confirm which interpretation applies to your appliance.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty dual-element boiler water temperature sensor Common
The most frequent cause. The boiler water temperature sensor used on many Viessmann models contains two independent sensing elements. When these elements age, become waterlogged, or suffer internal degradation, their readings drift apart. Once the difference exceeds the control board's tolerance, the F6 lockout is triggered. Replacing the sensor is typically the fix.
- Damaged or loose wiring to the temperature sensor Sometimes
Corroded terminals, a loose plug connector, or chafed cable insulation between the sensor and the PCB can introduce resistance into the circuit, causing the control board to see erroneous or fluctuating temperature values. This can mimic a failed sensor without the sensor itself being defective. An engineer will check wiring continuity before condemning the component.
- Faulty PCB or control unit Rare
If the sensor and its wiring both test correctly, the fault may lie with the control board itself misinterpreting the incoming signal. PCB faults are relatively uncommon but should be considered once the sensor and wiring have been ruled out by an engineer using a multimeter and diagnostic tools.
How to fix it
- Check the boiler display and note the exact code shown DIY safe
Confirm the display reads F6 and not a similar-looking code such as F4 or F5. Take a photo of the display for reference when speaking to an engineer. If your model shows a sub-code alongside F6, note that too, as it helps pinpoint the fault more quickly.
- Reset the boiler once or twice DIY safe
On most Vitodens models, press and hold the Reset (R) button on the front panel for approximately two seconds until the boiler attempts to restart. On touchscreen models, follow the on-screen reset prompt. If the boiler fires up and runs normally, monitor it over the next hour. If F6 returns immediately or after a short time, stop resetting — repeated resets will not clear a genuine sensor fault and can put unnecessary strain on ignition components.
- Check your boiler's system pressure DIY safe
While not a direct cause of F6, low system pressure (below around 1 bar) can cause secondary issues. The pressure gauge on the boiler front should read between 1 and 1.5 bar when cold. If it is low, top up via the filling loop and re-attempt a reset. If pressure is normal, proceed to the next step.
- Do not attempt to inspect or replace the temperature sensor yourself Gas Safe engineer
The boiler water temperature sensor is a sealed internal component. Accessing it requires draining part of the system, isolating the boiler, and working in close proximity to the gas valve and other safety-critical parts. This work must only be carried out by a Gas Safe registered engineer. Attempting it yourself will almost certainly void any remaining warranty and could create a safety hazard.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will use a multimeter to test the resistance of both sensor elements at a known temperature, checking whether the readings fall within Viessmann's specified parameters. They will also inspect wiring continuity and connector condition. If the sensor is faulty it will be replaced; if wiring is the issue, damaged sections will be repaired or re-terminated. For best results, ask for a Viessmann-accredited or Viessmann-trained installer, as they will have direct access to Viessmann technical support and genuine spare parts.
Parts you may need
- Dual-element boiler water temperature sensor (NTC thermistor) · from £45
- Sensor wiring harness / connector plug · from £25
- PCB (control board) — if sensor and wiring are ruled out · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£280, depending on the underlying cause.
Frequently asked questions
Can I keep resetting the boiler to get hot water while I wait for an engineer?
Resetting the boiler one or two times is reasonable to confirm whether the fault is intermittent, but if F6 keeps returning you should stop. The boiler is locking out because it genuinely cannot verify the water temperature, which means it cannot guarantee safe combustion. Repeatedly overriding a lockout risks damaging the ignition system and may cause the boiler to operate outside safe temperature limits. Leave it locked out and arrange for an engineer promptly.
How much does it typically cost to fix a Viessmann F6 fault in the UK?
For most homeowners, fixing F6 means a sensor replacement, which usually comes to around £120–£200 once you include the engineer's call-out or diagnostic fee (often around £90–£100 on its own), the part, and the labour to fit it. If the issue turns out to be a wiring fault rather than the sensor itself, costs can be at the lower end of that range. In the rare scenario where the PCB is at fault, expect to pay significantly more — potentially £350–£500 or above — though this outcome is uncommon. If your boiler is still under Viessmann's warranty, the repair may be covered at no cost, so check your documentation before booking an independent engineer.
Why does the F6 code mean different things on different Viessmann boilers?
Viessmann uses a shared fault-code numbering system across a wide product range, but the underlying hardware and software varies between models. On condensing boilers fitted with a dual-element NTC sensor — such as most Vitodens wall-hung models — F6 specifically flags a discrepancy between the two sensor elements. On a small number of older or different-range models, the same code can relate to a gas-pressure or gas-valve signal issue. This is why Viessmann's own fault-code checker asks you to select your exact model first. When in doubt, quote both your boiler model number and the F6 code to the engineer so they can pull up the correct diagnostic flowchart.
My Viessmann boiler is over 10 years old and keeps throwing fault codes — should I repair or replace it?
A one-off F6 fault on an otherwise reliable boiler is usually worth repairing, especially if the fix is simply a sensor replacement. However, if your boiler is over 10–12 years old and has had several different faults in the past couple of years, repair costs can quickly add up. A modern Viessmann condensing boiler can achieve efficiencies above 90%, so replacing an ageing appliance often pays back in lower gas bills over three to five years. Ask your engineer for an honest assessment of the boiler's overall condition before committing to an expensive repair on an old unit.