Viessmann b8 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann b8 fault code mean?
The B8 code on Viessmann boilers signals a 'Burner Blocked' condition, most commonly caused by a fault with the flue gas temperature sensor or its wiring. When the boiler's controls detect that the sensor cable has broken, come loose, or that the sensor itself has failed, it blocks burner operation as a precaution. Unlike some other Viessmann fault codes, B8 does not always require a manual reset — if the underlying problem is resolved, the burner can resume automatically. It is worth noting that on certain Vitopend models, B8 can instead point to electromagnetic interference (EMC) disrupting the boiler's electronics, though across the wider Vitodens range the flue gas sensor fault is the primary and most frequently documented cause.
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
- Broken or disconnected sensor cable Common
The wiring loom connecting the flue gas temperature sensor to the control board can develop a break, become frayed at a connection point, or work loose over time — particularly if the boiler has experienced vibration or repeated thermal cycling. This interrupts the signal entirely and triggers the B8 code.
- Failed flue gas temperature sensor Common
The sensor itself is an NTC thermistor whose resistance changes in proportion to temperature. Years of exposure to hot flue gases gradually degrade the component, causing it to produce readings outside the expected range or fail altogether. An engineer can test resistance values to confirm whether the sensor has reached the end of its serviceable life.
- Sensor incorrectly seated in the flue Sometimes
If the sensor has shifted out of its correct position — perhaps after previous maintenance or during installation — it may not be reading flue temperatures accurately. The control board interprets out-of-range readings as a fault and blocks the burner.
- Electromagnetic interference (EMC) Rare
On certain Vitopend variants, B8 is specifically associated with a strong external electromagnetic field interfering with the boiler's electronics rather than a physical sensor failure. An engineer will look for nearby sources of interference such as certain electrical equipment or poor wiring earthing if the sensor and cable check out fine.
How to fix it
- Check that your gas supply is working DIY safe
Before assuming a boiler fault, confirm that other gas appliances in your home (such as a gas hob) are working normally. If there is no gas supply at all, contact your gas supplier rather than an engineer.
- Attempt a single boiler reset DIY safe
Locate the reset button on your Viessmann boiler — on most Vitodens models this is a flame symbol button on the control panel. Press and hold it for a few seconds. If the boiler fires up and runs normally, monitor it closely over the next day or two. Because B8 can clear automatically once a transient fault resolves, a single reset is reasonable. Do not reset more than two or three times in total; repeated resets without resolution indicate an underlying problem that needs professional attention.
- Do not attempt to open the boiler casing Gas Safe engineer
The flue gas temperature sensor and its wiring are inside the boiler casing. Inspecting, testing, or replacing these components requires a Gas Safe registered engineer. Opening the casing yourself risks injury, invalidates your warranty, and may be unlawful for gas-connected components.
- Engineer inspects and tests the sensor cable Gas Safe engineer
A Gas Safe engineer will open the casing and trace the sensor wiring for any visible breaks, fraying, or loose terminal connections. A damaged cable is often the quickest and least expensive fix — repairing or replacing just the wiring before moving on to the sensor itself.
- Engineer tests the NTC thermistor sensor Gas Safe engineer
Using a multimeter, the engineer checks the electrical resistance of the flue gas temperature sensor against Viessmann's specified values for a given temperature. A reading outside the expected range confirms the sensor has failed and needs replacing.
- Engineer checks sensor positioning in the flue Gas Safe engineer
If the cable and sensor both test correctly, the engineer will verify that the sensor is correctly seated in its designated location within the flue assembly. Repositioning a displaced sensor can resolve the fault without any parts being replaced.
- Engineer checks for electromagnetic interference (Vitopend models) Gas Safe engineer
Where the boiler is a Vitopend variant and no sensor or wiring fault is found, the engineer will look for nearby sources of strong electromagnetic fields — such as certain industrial equipment, poorly earthed wiring, or large motors — that could be disrupting the boiler's electronics.
- Control board replacement if fault persists Gas Safe engineer
If all sensor and wiring checks come back clear and the B8 code continues to appear, the boiler's control board (PCB) may be misreading the sensor signal. PCB replacement is a last resort and must be carried out by a Gas Safe engineer. Get a quote before proceeding, as this is the most expensive repair option.
- Call a Gas Safe registered engineer Gas Safe engineer
If a single reset does not clear B8 for good, contact a Gas Safe registered engineer — ideally one with Viessmann experience or Viessmann Trained Installer accreditation. They will carry out a proper diagnostic inspection and advise on the most cost-effective repair.
Parts you may need
- Flue gas temperature sensor (NTC thermistor) · from £65
- Sensor wiring loom / cable harness · from £35
- Control board (PCB) · 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–£320, depending on the underlying cause.
Frequently asked questions
Do I need to manually reset my Viessmann boiler to clear a B8 code?
Not necessarily. B8 is unusual compared with some other Viessmann fault codes in that the boiler can resume operation automatically once the root cause is corrected — for example, if a loose sensor connection is reconnected. That said, it is reasonable to try one reset yourself. If the code returns after resetting, stop resetting and arrange for an engineer to investigate, as repeated resets without a fix can mask a developing fault.
How much does it typically cost to fix a Viessmann B8 fault in the UK?
For most homeowners, fixing a B8 fault means replacing the flue gas temperature sensor and/or its wiring. All-in (parts and labour including the engineer's callout), expect to pay in the region of £120 to £320. A straightforward cable repair or sensor swap sits at the lower end; if the engineer's time is longer or a higher-specification sensor is needed for your specific model, costs rise toward the upper end. In the rare situation where the control board (PCB) needs replacing, costs can be considerably higher — often £400 or more including labour — so it is worth getting a quote and weighing that against the age of the boiler before proceeding.
Can a B8 code on a Vitopend mean something different to a Vitodens?
Yes, and this is worth knowing. On the Vitodens range (100-W, 200-W), B8 almost always points to the flue gas temperature sensor or its wiring. On certain Vitopend models, Viessmann's service documentation specifically links B8 to electromagnetic interference (EMC) disrupting the boiler's electronics rather than a physical sensor failure. If you have a Vitopend and the sensor checks out fine, ask your engineer to investigate potential sources of strong electromagnetic fields near the boiler.
Will my Viessmann warranty cover a B8 repair?
It depends on your warranty status and service history. Viessmann requires annual servicing to keep extended warranties valid — if services have been missed, your warranty protection may have lapsed, leaving you responsible for the full repair cost. Check your warranty documents and, if in doubt, contact Viessmann UK or your installer directly before authorising any repair work.