YELLOW LED ON every 0.5-secs Low

Vokera 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 1 Vokera model in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

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.

Typical repair cost £0-80 (including labour)

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

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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 27 April 2026