Boiler locks out intermittently, especially in very cold weather, with gurgling sounds in the boiler.

Safety first: Switch off the boiler before working on the condensate pipe.

• Inspect external pipe: Look for ice or bulging on the plastic condensate pipe outside.
• Thaw gently: Pour warm (not boiling) water along the pipe and around the outlet. Do not use a blowtorch or open flame.
• Reset boiler: Once thawed, reset and observe operation.
• Preventive measures: Insulate the pipe, increase its diameter, or reroute internally (engineer task) to reduce freezing risk.