This fault can involve incomplete combustion and carbon monoxide.
If anyone in the property feels sick, dizzy or drowsy, or you can smell gas: open the windows, leave the building, and do not switch anything on or off. Then call the National Gas Emergency Service.
0800 111 999Viessmann 0A fault codewhat it means and how to fix it
On the B1HC-generation 100-W and 111-W, 0A means a fitted CO limiter has responded to carbon monoxide, or a fitted gas pressure switch has seen gas pressure fall. Switch off; Gas Safe engineer.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Viessmann 0A
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 0A means
0A appears on the Vitodens 100-W B1HC/B1KC and 111-W B1LD (2016 to 2020) and carries two rows in Viessmann's table, both marked "if installed". The first is "Burner blocked — CO limiter (if installed) has responded. CO concentration too high. — Check the heating system. Remove the cause of escaping CO." The second is "Burner blocked — Gas pressure switch (if installed) has responded. Gas pressure too low. — Check the gas supply." Neither device is standard on a UK domestic installation — a CO limiter is an optional accessory, and a gas pressure switch is normally fitted where the supply is marginal or shared — so 0A on a boiler that has neither fitted should not appear. On one that has, the first row is the only code in the whole Viessmann two-character table that names carbon monoxide, and Viessmann's measure is not to reset it but to remove the cause of the escaping gas.
Because the burner is blocked rather than locked out, the boiler may attempt to run again if the condition clears, which with a CO limiter is not a reassurance. Treat 0A as you would a CO alarm: switch the boiler off, ventilate, and do not run it again until a Gas Safe engineer has found the cause. If your installation has a gas pressure switch and no CO limiter, the code means the supply pressure dropped below the switch setting while the burner ran — a governor, a shared supply under load, or an LPG regulator — and the engineer's check is the gas supply. The code is absent from every earlier two-character unit and from the 050-W BPJD. On the 2020-on numeric-table units the equivalents are F.352 (combustion CO limit exceeded) and F.368 (gas pressure switch fault).
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Carbon monoxide detected by the fitted CO limiter | Products of combustion escaping into the room — a flue leak, spillage from a compartment, or a heat exchanger fault. Viessmann: remove the cause of escaping CO. | Occasional |
| Gas pressure too low at the fitted gas pressure switch | Supply pressure fell below the switch setting while the burner ran. A governor, a meter, a shared supply under load, or an LPG regulator or empty cylinder. Viessmann: check the gas supply. | Occasional |
| Limiter or switch wiring | A device fitted but wired wrongly, or a lead that has failed, produces 0A with nothing wrong in the room or the supply. Diagnosed after the two real causes are excluded, never before. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£70–£300
Time on site
1h 30m
A gas supply check is a call-out; a flue repair or a heat exchanger inspection is the higher figure. A CO alarm, if you do not have one, is under £30 and is the thing to buy first.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Own this appliance and had this code? . Nothing is published until a person has read it.
Related Viessmann fault codes
- F4Stop — call an engineerNo flame signal was detected during burner start on any two-character Vitodens, so the burner has locked out. Reset once; if it returns, call a Gas Safe engineer.Related: ignition and gas supply
- F.65Stop — call an engineerNo flame signal at burner start on a 2020-on Vitodens — the numeric table's equivalent of F4. Burner locked out. Check the gas is on, reset once; if it returns, call a Gas Safe engineer.Related: ignition and gas supply
- F3Stop — call an engineerThe control unit saw a flame signal before it had opened the gas — a flame where there should be none, or a false signal from the ionisation electrode. Lockout; Gas Safe engineer.Related: ignition and gas supply
- E5Get it checkedViessmann call E5 an "internal fault" but their only instruction is to check the ionisation electrode and its cables — a flame-monitoring fault, blocked not locked, and one for a Gas Safe engineer.Related: ignition and gas supply
- F.357Get it checkedInsufficient gas supply on a 2020-on Vitodens. F.353 is the softer version with output reduced; F.467, F.383 and F.384 blame calibration or a contaminated gas line. Check the gas is on, then call an engineer.Related: ignition and gas supply
- F.359Stop — call an engineerNo ignition spark on a 2020-on Vitodens, so the burner locked out before it could light. Ignition electrode, ignition module or heat management unit: a Gas Safe engineer's diagnosis.Related: ignition and gas supply
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.