Error L1

Safety precautions:

1. Do not remove the boiler casing or access internal components unless you are a Gas Safe registered engineer. Working on gas and electrical parts is dangerous. 2. If you smell gas, evacuate the property immediately and contact your gas emergency number. 3. Before carrying out any checks isolate electrical supply to the boiler only if instructed by the manual for a specific task and only if you understand the risks; many simple checks do not require isolating electricity but do not open the unit.

Initial homeowner checks (safe and quick):

1. Note the exact display and any additional symbols. Press the Restart button once to attempt a normal restart. If the fault clears and stays cleared, monitor the boiler for recurrence. 2. Check system pressure on the pressure gauge. It should normally be around 1.0 to 1.5 bar when cold. If below 1.0 bar, re-pressurise the system using the filling loop per the boiler manual until pressure is about 1.0–1.5 bar, then close the loop. 3. Bleed radiators to remove trapped air, then re-check and re-pressurise the system to 1.0 bar. 4. Ensure all isolation valves to the boiler and on flow/return are fully open and that the programmer/room thermostat is calling for heat. 5. Reduce the central heating target temperature on the boiler control to a lower setting and see if the boiler runs without fault. 6. Check the external condensate pipe (outside) for ice or blockage. If frozen, thaw carefully (warm water) and then restart the boiler. 7. Listen to the boiler during a call for heat: you should hear the pump running. If the pump is silent or making unusual noises, note this for the engineer.

Further diagnostic steps a homeowner can safely perform and observe:

1. After completing the checks above, press Restart and observe whether the boiler attempts to ignite and run. If it locks out again quickly with L1, this indicates persistent overheat or no circulation. 2. Check whether radiators heat normally or only warm at the top/bottom; this helps indicate circulation problems or air locks. 3. Monitor the system pressure for a fall after restarting; a dropping pressure can indicate a leak which needs professional attention.

When to call a professional and what they will check:

1. Call a Gas Safe registered engineer if L1 persists after the safe homeowner checks, or if you find a frozen condensate pipe you cannot clear, a silent or noisy pump, visible leaks, or repeated lockouts. 2. The engineer will isolate and open the boiler, check pump operation and pump rotor (seized rotor or air-locked pump), verify correct pump direction and speed, inspect and test flow and return thermistors for correct resistance/operation, and measure actual flow and return temperatures. 3. They will check for blockages in the heat exchanger, limescale or sludge in the system, any closed or blocked valves, and correct wiring or reversed flow/return connections. 4. If a sensor or thermostat is faulty (flow thermistor, return thermistor), they will test and replace as required. If control electronics or safety thermostats have tripped or failed, the engineer will advise on repair or replacement of control box components. 5. If the condensate trap, flue thermostat, or flue path is implicated, the engineer will inspect and clear or replace components.

Important notes:

1. If the boiler repeatedly locks out with L1 do not repeatedly try to force it to run; persistent overheating risks component damage. 2. Do not attempt internal repairs, remove the burner, heat exchanger or electrical connections yourself. Any internal work, thermistor replacement, pump replacement, or gas/electrical repairs must be done by a qualified engineer. 3. After any repair or service the engineer should recommission and test the boiler, verify correct pressures, correct circulation, and that the fault no longer occurs.

If the above homeowner actions do not clear the fault, or if you are unsure at any point, contact a Gas Safe registered engineer (or RGII in Ireland) to diagnose and fix the underlying cause.