0AViessmann
Viessmann 0A: CO limiter has responded (CO concentration too high), or gas pressure switch has responded (gas pressure too low)
Checked against Viessmann's own documentation: Viessmann Vitodens 100-W (B1HC/B1KC) installation and service instructions, 5697 787 GB. Last reviewed October 2026. How we check
Engineer only
Viessmann describes 0A on Vitodens 100-W (B1HC/B1KC) boilers as: cO limiter has responded (CO concentration too high), or gas pressure switch has responded (gas pressure too low).
What you can safely do
There is nothing you can safely do yourself with this one. Leave the boiler off and book an engineer.
Before you book an engineer
- Write down the code exactly as the display shows it: 0A.
- Find your boiler's model name. It is usually on the front panel or in the user manual. The table below shows why it matters: Viessmann codes can mean different things on different models.
- Note when it started and what the boiler was doing (heating, hot water, or starting up).
- If the boiler is still under warranty, check your warranty paperwork before you book anyone.
Call a Gas Safe engineer if
- Book a Gas Safe registered engineer.
- If a carbon monoxide alarm is sounding, or anyone feels unwell (headache, dizziness, nausea), get fresh air, leave the boiler off and call 0800 111 999.
Find a Gas Safe engineer near you
Opens the official Gas Safe Register search.
Applies to Vitodens 100-W (B1HC/B1KC). Codes can differ between models, so check your manual.