Short answer
A lockout is the boiler protecting itself after failing to detect a stable flame, seeing an unsafe temperature or losing air proving. Common causes are worn ignition or flame-sensing electrodes, blocked condensate traps, low gas inlet pressure at full fire, failing fans or air pressure switches, and poor water flow.
What to check
Record the fault code and frequency. An engineer checks gas inlet pressure at full fire, electrode condition and flame signal, condensate drainage, fan and air proving, and water flow before replacing parts.
Pressure link
Lockouts that only occur when several boilers fire together usually point to gas pressure loss in the plant room supply rather than a boiler fault.
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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