Short answer
A burner locks out when its flame safeguard fails to prove a safe start or loses the flame — usually from low or unstable gas pressure, a faulty ignition or flame detection, air proving or fan problems, valve proving failure, or flue and air supply issues. One reset is acceptable; a repeat lockout needs an engineer.
How a lockout happens
Every commercial burner runs a fixed sequence: pre-purge, ignition, safety time to prove the flame, then run. If the flame isn't detected within the safety time, or air pressure, valve proving or flame supervision fails, the control shuts the gas valves and locks out until it is manually reset.
Common causes
Low gas inlet pressure when several appliances fire together; dirty or misaligned ignition electrodes and flame probes or UV cells; failed ignition transformers; air pressure switch or fan faults; valve proving (VPS) failures from a passing gas valve; blocked condensate traps; and flue recirculation or poor combustion settings.
What we check
We read the control's fault history, measure gas pressures at the meter and burner under load, check flame signal strength, test ignition and air proving, prove the gas valves and finish with combustion analysis at high and low fire.
Before you reset
A single reset after a power cut or a known one-off event is reasonable. Repeated resets are not: many commercial controls count resets and lock out further (for example a 'too many resets' code), and resetting a flame or gas valve fault over and over can mask a dangerous condition. If a lockout returns, isolate the appliance and call a Gas Safe registered engineer with the relevant commercial competence.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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