Viessmann b7 fault codewhat it means and how to fix it
On the B1HC-generation 100-W and 111-W, b7 is emergency mode after a control unit fault and Viessmann say to switch the boiler off and on. On a Vitotronic unit it is a coding card fault.
Take care
Which display does your Viessmann have?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Which display does your Viessmann have?
Emergency mode after a burner control unit fault. Viessmann: turn the ON/OFF switch off and on again. The boiler is running on a fallback until then.
Switch the boiler off and on once. If b7 comes back, book a Gas Safe engineer; the control unit is the suspect.
Which display does your Viessmann have?
Coding card fault: the plug-in card holding the boiler's parameters is missing, unseated or faulty. Viessmann: plug in the coding card or replace it if faulty.
A contractor job — the card is inside the control unit. If Fd is shown with it, the card is missing rather than faulty.
What b7 means
b7 exists on two families of control unit and does different work on each. On the Vitodens 100-W B1HC/B1KC and 111-W B1LD (the 2016 to 2020 touch-panel units) the row is "Emergency mode — Burner control unit fault — Turn the ON/OFF switch off and on again." Emergency mode means the boiler is still running, on a fixed fallback rather than under normal control, and the burner control unit has logged a fault in itself that a restart is expected to clear. It is the one two-character code on the 100-W for which Viessmann's published remedy is a power cycle, and because it is printed in the installation and service instructions rather than the operating instructions, we quote it rather than recommend it: the ON/OFF switch on the front of the boiler is described to the householder in the operating instructions, so using it is within what Viessmann publish, but the diagnosis behind the code is the contractor's.
On the Vitotronic-controlled 200-W B2HA and B2HB, 222-F B2TA and 242-F B2UA the row is "Burner blocked — Coding card fault (boiler coding card fault) — Plug in coding card or replace if faulty." The coding card is the plug-in module carrying the parameters for the specific boiler; the control unit will not run without a valid one, and b7 usually appears alongside Fd on the B2HA when the card is missing altogether. The WB1A, WB1B, WB1C and B1HA tables have no b7 at all. On the 2020-on numeric-table units the closest rows are F.454 (incorrect parameter set of HBMU heat management unit) for the coding-card sense and F.70 for the control unit sense.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Burner control unit self-check fault (B1HC, B1LD) | The unit has dropped to emergency mode after an internal fault. Viessmann expect a restart to clear it; a recurring b7 is a control unit on its way out. | Common |
| Coding card unseated or faulty (Vitotronic units) | The card has worked loose in its socket, has been left out after a control unit swap, or has failed. Refit or replace. | Common |
| Wrong coding card for the boiler (Vitotronic units) | A card from a different output or model. The FE row on the same units names "incorrect coding card"; b7 can precede it. | Rare |
What to try yourself
On a B1HC/B1KC or 111-W B1LD only: switch the boiler off at the ON/OFF switch on the front, wait a few seconds, and switch it on again. This is Viessmann's published measure for b7 on these units.
On a Vitotronic 200-W, 222-F or 242-F, b7 is a coding card fault and switching off will not help; the card is inside the control unit and is a contractor job.If b7 returns after the restart, note it and call your heating contractor. Do not keep power-cycling a boiler that repeatedly drops into emergency mode.
Get a Gas Safe registered engineer to look at your Viessmann b7
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£70–£500
Time on site
1 hour
A restart or a reseated coding card costs a call-out at most. A control unit, if the B1HC fault persists, is the high figure.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Viessmann burner control unit (control unit PCB / HBMU heat management unit)
£250–£500Authorised Viessmann parts distributor
Price checked 28 September 2026
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Viessmann fault codes
- F0Get it checkedInternal fault in the burner control unit. Viessmann's only measure on the two-character units is "Replace control unit"; the Vitotronic B2HB allows one reset first. Not a householder repair.Related: electronics and power
- F.59Get it checkedMains undervoltage on a 2020-on Vitodens: the burner locks out and the pump stops until the supply recovers. If the voltage is good and it recurs, Viessmann replace the fan unit.Related: electronics and power
- F.70Get it checkedAn internal fault in the HBMU heat management unit of a 2020-on Vitodens. Nearly twenty numbers say the same: reset once, and if it returns the unit is replaced. Engineer only.Related: electronics and power
- FdGet it checkedBurner control unit fault, which Viessmann first blame on the ignition electrodes, their cables or electrical interference, and only then on the unit. FF is the same row. Engineer job.Related: electronics and power
- FFGet it checkedOn the two-character units FF is the same burner control unit fault as Fd. On a Vitotronic 200-W it means an internal fault or a disabled reset key, and Viessmann say restart the appliance.Related: electronics and power
- 0CGet it checkedMains voltage too low. The burner is blocked while the supply is out of range and restarts on its own when it recovers. Viessmann's measure on the WB1A is to contact the electricity supplier.Related: electronics and power
Own a Viessmann boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.