Viessmann 0C fault codewhat it means and how to fix it
Mains 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.
Take care
What 0C means
0C is "Burner blocked — Mains voltage too low — Check the power supply" on the Vitodens 100-W B1HC/B1KC, the 111-W B1LA, B1LB and B1LD, and in older wording on the first-generation WB1A: "Voltage too low — Contact the electricity supplier." The burner control unit monitors its own supply and will not run the burner if the voltage is below what the fan, gas train and ignition need to behave predictably. The block clears by itself when the voltage recovers, so 0C is typically seen during a brown-out, on a supply shared with a heavy load starting up, or on an installation with a long, undersized cable run to the boiler. It is not in the WB1B, WB1C or B1HA tables — on those units the same event surfaces as an unexplained restart or, if the control unit is upset by it, as F0 — and not on the 050-W BPJD.
A 0C that appears once during a storm needs nothing. One that recurs at the same time each day, or whenever a particular appliance starts, is a supply problem rather than a boiler problem, and Viessmann's WB1A measure — contact the electricity supplier — is the right first call if the lights in the house dim at the same time. If the supply proves sound at the meter, the wiring between the consumer unit and the boiler, or a fused spur with a poor connection, is the heating contractor's to check. On the 2020-on numeric-table units the equivalent is F.59 (power supply, undervoltage), which locks the burner out and stops the pump until the supply returns.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Low mains voltage from the supply | A brown-out or a supply running at the bottom of its tolerance at peak times. If lights dim when 0C appears, this is it, and it is the electricity supplier's to fix. | Common |
| Poor connection in the boiler's circuit | A loose terminal in the fused spur or isolator, or a long undersized cable, drops voltage under load. The heating contractor's check after the supply is proven. | Common |
| Heavy load starting on the same circuit | A shower, a pump or a workshop machine on a shared circuit pulling the voltage down momentarily as it starts. | Occasional |
| Control unit supply monitoring | A control unit reporting low voltage with a good supply at its terminals is faulty. Rare, and proven by measurement. | Rare |
What to try yourself
Notice what else is happening when 0C appears. If lights dim or flicker at the same time, or it coincides with a large appliance starting, the cause is the supply. Viessmann's own instruction on the WB1A is to contact the electricity supplier; your supplier has an emergency number for voltage problems.
If the house supply seems normal and 0C keeps returning, note when it happens and call your heating contractor. The burner restarts on its own when the voltage recovers, so there is nothing to reset.
Do not open the boiler casing or the fused spur. Checking the supply at the boiler is a live measurement for a qualified person.
Get a Gas Safe registered engineer to look at your Viessmann 0C
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
£0–£150
Time on site
20 minutes
A supply fault costs nothing to report. A re-made spur or a replaced isolator is a short electrical visit.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
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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
- b7Get it checkedOn 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.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.