What the manual says
Manufacturer manualVenting program (constant temperature control unit)
From this model's manual
Verified against the Viessmann/Viessmann Vitodens 333 Ws3a 5862566 installation & service manual.
DIY fix possible
Wait for the process to complete automatically, which usually takes around 20 minutes.Do not switch the boiler off while the venting program is active.If the code does not clear after an hour, turn the boiler off and back on at the fused spur to restart.
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All 75 fault codes on the Vitodens 333 WS3A
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