The boiler has locked out because the fan is not reaching or holding the speed it expects.
Most common on: Logic, Logic+, Vogue
L5 is a fan-related lockout on Ideal boilers. The control board monitors the fan speed feedback, and if the fan does not reach the expected speed — or drops out of range during operation — the boiler locks out rather than run combustion without a properly working fan. It is closely related to the F3 fan fault; L5 is more specifically the lockout state.
As the fan governs both combustion air and flue clearance, this is a safety lockout. A single L5 may clear with a reset, but a boiler that keeps locking out on L5 has a fan, wiring or flue problem that needs a Gas Safe engineer.
Hold the reset button. If the boiler fires and runs normally, it may have been a one-off. If L5 returns, do not keep resetting.
Look at the external flue terminal for obvious blockages such as debris or nesting. Do not attempt to take the flue apart.
On start-up you should hear the fan spin up. A fan that is silent, noisy or laboured points to a failing fan or its wiring.
Fan speed, wiring, air-pressure sensing and the PCB all need a Gas Safe engineer with the correct test equipment — this is not a homeowner repair.
Work on the gas side of a boiler must be carried out by a Gas Safe registered engineer. The checks above are limited to safe, non-invasive things a homeowner can look at (pressure, resets, the gas supply) — if in doubt, stop and call an engineer.
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The boiler has locked out because the fan is not reaching or holding the speed it expects. L5 is a fan-related lockout on Ideal boilers. The control board monitors the fan speed feedback, and if the fan does not reach the expected speed — or drops out of range during operation — the boiler locks out rather than run combustion without a properly working fan. It is closely related to the F3 fan fault; L5 is more specifically the lockout state.
You can try a single reset and the safe checks below, but Ideal L5 usually needs a Gas Safe registered engineer. Never work on the gas side of a boiler yourself — it is illegal and dangerous unless you are Gas Safe registered.
A recurring L5 means the underlying cause has not been fixed — most often worn, seized or failing fan. Resetting repeatedly will not solve it; get the boiler diagnosed properly.
See all Ideal fault codes.