Error L4 or F4

Safety first

1) If you smell gas, leave the property immediately, do not operate electrical switches, ventilate doors/windows if safe, and call the gas emergency number. 2) If you are not confident working around boilers or plumbing, stop and call a Gas Safe registered engineer. 3) Do not attempt internal electrical or gas repairs, and do not remove sealed casing or safety guards on the boiler unless you are qualified.

Initial quick checks a homeowner can do

1) Note the error code and any other messages. Take a photo of the display and write down boiler model and serial. 2) Try a single reset: press the boiler RESTART button or switch the boiler off at the fuse spur for 20–30 seconds and switch back on. Do not repeatedly reset the boiler more than 2 times. 3) Check the system pressure on the boiler gauge. It should normally be around 1–1.5 bar when cold for most system boilers. If pressure is low (below about 1.0 bar), you can re-pressurise using the filling loop following the boiler manual instructions. After repressurising, try a reset.

Checks for circulation problems (safe, non-invasive checks)

1) Check all radiator and boiler isolation valves are open. Ensure motorised zone valves (if present) are not closed. 2) Bleed the radiators to remove trapped air that can prevent pump circulation. After bleeding, re-check and top up system pressure if it drops. 3) Listen or feel (carefully, without touching moving parts) near the boiler for the pump running when the CH demand is on. If the pump is silent when CH should be running, circulation may be the issue—do not force the pump or remove covers; call an engineer.

Simple thermistor/pipe checks (only if accessible without removing boiler casing)

1) If the flow thermistor is externally visible on pipework near the boiler, check visually that the small sensor is clipped firmly to the flow pipe and the clamp is in good condition. A loose or incorrectly positioned clamp can give wrong readings. 2) Check visible wiring for obvious damage or loose plug connections at accessible terminals (do not open the boiler casing). If a connector is loose in an easily accessible external terminal and you are competent, you can reseat it; otherwise leave it for an engineer.

When to call a professional (and what they will likely check)

1) If the fault remains after resets, repressurising, bleeding radiators and ensuring valves are open, call a Gas Safe registered engineer. 2) Tell the engineer the exact fault code (L4/F4), what you have already tried, the boiler model and serial number, and whether any other faults or unusual noises are present. 3) A professional will diagnose by measuring thermistor resistance, checking the thermistor clamp position, testing wiring continuity and connections, checking the pump operation and flow rates, verifying system pressure and any airlocks, and testing the PCB. They can replace a failed thermistor, repair wiring, replace the pump or PCB as necessary.

Additional notes and precautions

1) Do not attempt to replace the thermistor, PCB, or perform gas/pump/PCB repairs yourself unless you are a qualified Gas Safe engineer. 2) Keep a record of fault occurrences, times and any resets you performed—this helps the engineer diagnose intermittent faults. 3) If your boiler is under warranty, contact the manufacturer or installer before commissioning work. 4) Avoid repeated resets; if the fault persists after one or two resets and simple checks, stop and arrange a visit from a qualified engineer.

Summary

Try one reset, confirm system pressure is correct and radiators are bled and valves open. If the boiler still shows L4/F4, it is usually a thermistor, wiring or circulation fault that requires a Gas Safe engineer to correctly diagnose and repair.