BoilerFaultCodesTechnical description
Lock-out: maximum temperature rise is exceeded, dT between supply and return >= 22°C, rate of rise > 2,7°C/sec, or start with no flow/insufficient water. Also activated if shut down input (terminals 5 and 6) is opened.
What causes this fault?
This fault occurs when heat builds up inside the boiler faster than the system can carry it away to your radiators. The most common reasons are a failed internal pump that isn't pushing water through the pipes or a physical blockage, such as sludge or debris, restricting the flow. Essentially, the boiler is protecting itself from overheating because the water is essentially standing still while being heated.
This issue often comes to light in autumn when the heating is turned back on after a long summer break, as pump components can seize during periods of inactivity.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.