BoilerFaultCodesVokera YELLOW LED ON every 0.5-secs Fault Code
Also written as YELLOWLEDONEVERY05SECS — same fault.
The Vokera YELLOW LED ON every 0.5-secs fault code means: your boiler is currently running in a special engineer-only testing mode used to check combustion levels, meaning it is not in its normal operating state.. It appears on 2 Vokera models in our database.
Technical description: CO function active
What causes the Vokera YELLOW LED ON every 0.5-secs fault?
This status is triggered when the selector dial has been turned to the 'CO' position, which is a dedicated mode for engineers to test if the gas is burning safely. It usually occurs because the dial was accidentally left in this position after a service or because the knob has been moved during cleaning. It is not an internal component failure, but rather the boiler obeying a specific manual command.
DIY fix possible
Locate the mode selector switch on the front control panelEnsure the selector is not left in the 'service' or 'test' positionTurn the selector dial to 'OFF' for a few seconds, then back to your desired 'Heating and Hot Water' setting
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Avoid the next call-out bill
A cover plan would handle faults like YELLOW LED ON every 0.5-secs: no call-out fees, no surprise bills.
Affiliate links. We may earn a commission at no extra cost to you.
Vokera models that show the YELLOW LED ON every 0.5-secs fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Vokera YELLOW LED ON every 0.5-secs fault code information on this page is sourced and cross-checked against the official Vokera service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026