Viessmann b0 Fault Code: Causes, Fixes & Repair Costs
What does the Viessmann b0 fault code mean?
The B0 fault code on a Viessmann boiler indicates a burner blockage triggered by a short circuit at the flue gas temperature sensor (internally logged as fault 141). When the boiler's control board detects an invalid signal from this sensor — caused by a short circuit rather than a normal resistance reading — it shuts the burner down as a precautionary measure. Unlike a full lockout, this is classified as a 'burner blocked' condition, meaning the boiler can resume operation automatically once the underlying fault is resolved, without necessarily requiring a manual reset. On some Viessmann models, particularly certain Vitopend variants, the same underlying fault may appear as Fb0 rather than B0, but the meaning and diagnosis are essentially the same.
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 flue gas temperature sensor (thermistor) Common
The flue gas temperature sensor works by changing its electrical resistance in proportion to temperature — as temperature rises, resistance falls, and the PCB reads this to manage burner output. A short circuit within the sensor itself sends an erroneous signal to the control board, which immediately halts the burner. This is the most frequent root cause of a B0 code and typically requires the sensor to be replaced.
- Damaged or shorted sensor wiring or connections Common
Even if the sensor itself is sound, a damaged cable, corroded plug, or loose connection anywhere along the sensor's wiring loom can create a short circuit. Vibration, moisture ingress, or age-related cable degradation can all be responsible. An engineer will inspect the full wiring circuit before condemning the sensor.
- Incorrectly positioned or seated sensor Sometimes
If the flue gas temperature sensor has moved out of its correct position — perhaps following a previous service or repair — it may produce readings that the PCB interprets as a short. Repositioning and re-seating the sensor correctly can sometimes resolve the fault without any parts being needed.
- Fan not at standstill or electrical connection error Sometimes
In some cases, the B0 code can be triggered when the boiler's fan has not come to a complete standstill between cycles, or when there is a plug-in connection error on the control board side. An engineer should inspect the fan operation and all relevant electrical connectors as part of the diagnostic process.
- PCB fault Rare
If the control board itself has developed a fault — for example, a damaged input circuit for the flue sensor — it may misread or short the sensor signal internally, producing a B0 code even when the sensor and wiring are perfectly healthy. PCB faults are relatively uncommon but should be considered if all other components check out correctly.
How to fix it
- Check whether the boiler self-recovers DIY safe
Because B0 is a 'burner blocked' rather than a full lockout condition, the boiler may resume operation on its own once the fault trigger passes. Observe whether heating and hot water return without any action on your part. If the boiler does recover, monitor it closely over the next day or two — a recurring B0 suggests an intermittent fault that still needs professional attention.
- Attempt a single boiler reset DIY safe
If the boiler has not self-recovered, locate the reset button (usually marked with a flame symbol or 'Reset' on the control panel — check your user manual if unsure). Press and hold for the duration specified in your manual. Only attempt this once or twice at most. Repeated resets on a fault that keeps returning will not fix the underlying problem and could cause additional stress to components.
- Do not attempt to open the boiler or inspect internal components Gas Safe engineer
The flue gas temperature sensor, its wiring, and all connected components are inside the boiler casing. Opening the boiler and handling internal electrical or gas-related parts must only be carried out by a Gas Safe registered engineer. Attempting this yourself risks personal injury, could void your Viessmann warranty, and is potentially illegal if gas systems are disturbed.
- Book a Gas Safe registered engineer familiar with Viessmann boilers DIY safe
Contact a Gas Safe registered engineer — ideally one with experience of Viessmann systems, or a Viessmann-approved engineer if your boiler is still under warranty. You can find Gas Safe engineers at gassaferegister.co.uk. If your boiler is within its warranty period (Viessmann typically offer five years for registered installations with annual servicing), contact Viessmann UK directly on 01952 675000 before booking a third-party engineer, as warranty repairs may be covered.
- Engineer to inspect and test the flue gas temperature sensor and wiring Gas Safe engineer
The engineer will use a multimeter to measure the resistance of the flue gas thermistor and compare it against Viessmann's specified values for a given temperature. They will also inspect the sensor's wiring loom, connectors, and plug-in connections for signs of damage, corrosion, or shorting. Fan operation and electrical connections to the control board will also be checked as part of a thorough diagnosis.
- Replace the flue gas temperature sensor or repair wiring as required Gas Safe engineer
If the sensor is confirmed faulty, the engineer will fit a replacement Viessmann flue gas temperature sensor. If the wiring or a connector is damaged, these will be repaired or replaced. In the rare event that the PCB is found to be misreading the sensor signal, the control board may need to be replaced — your engineer will advise if this is the case.
Parts you may need
- Viessmann flue gas temperature sensor (thermistor) · from £55
- Sensor wiring harness / connector plug · from £25
- Viessmann PCB (control board) · from £290
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–£300, depending on the underlying cause.
Frequently asked questions
Is the Viessmann B0 fault dangerous — do I need to turn the boiler off immediately?
B0 is a safety-response code rather than an emergency gas leak or carbon monoxide warning. The boiler shuts the burner down automatically, so it is not operating unsafely. You do not need to cut the gas supply or evacuate your home. However, you should not repeatedly reset the boiler if the fault keeps returning — arrange for a Gas Safe engineer to inspect it promptly, particularly in cold weather when you are relying on heating.
What is the difference between B0 and Fb0 on a Viessmann boiler?
Both codes point to the same underlying fault: a short circuit at the flue gas temperature sensor causing the burner to be blocked. B0 is the code displayed on models such as the Vitodens 200-W, while Fb0 appears on some Vitopend variants. The diagnosis and repair process is identical for both — an engineer will test the sensor, wiring, and connections in the same way regardless of which version of the code your display shows.
How much does it typically cost to fix a Viessmann B0 fault code in the UK?
For the most common repair — replacing the flue gas temperature sensor and checking the wiring — most homeowners pay somewhere between £120 and £300 including parts and labour. The sensor itself is a relatively modest component (roughly £30–£80 for the part), and the labour time is usually an hour or two. If the fault turns out to involve the PCB rather than the sensor, costs can rise significantly — a replacement Viessmann control board can cost £200–£400 for the part alone, pushing the total repair bill higher. An engineer will be able to give you a firm quote once they have diagnosed the exact cause.
Will my Viessmann warranty cover a B0 fault code repair?
It may do. Viessmann typically offer a five-year warranty on boilers that were installed by an approved engineer and registered within 30 days of installation, provided the boiler has been serviced annually. If your boiler meets these conditions, contact Viessmann UK on 01952 675000 before booking an independent engineer, as a warranty claim could cover the cost of parts and labour. Using a non-approved engineer for repairs while the boiler is under warranty may invalidate your cover, so always check your warranty terms first.