Error L5 or F5

Safety precautions:

- If the boiler is displaying this fault, you can try a simple reset first: turn the boiler off at the boiler isolator or fused spur for 10–30 seconds and switch it back on. Do not repeatedly reset the boiler more than a few times.

- Always isolate electrical supply at the fused spur before opening the boiler case. If you need to isolate gas, turn the yellow gas isolation valve under the boiler to off (handle across the pipe). If you are not 100% confident working near live electrical parts or gas valves, stop and call a Gas Safe engineer.

- Do not touch hot pipes or components; let the boiler cool down before touching pipe thermistors or wiring.

Initial homeowner checks you can safely do:

1) Reset the boiler once as above and note whether the fault returns immediately or after a short period when the boiler tries to fire. If it clears and stays off, monitor for recurrence.

2) Check system water pressure: look at the pressure gauge and ensure the system is within the normal range (usually about 1.0–1.5 bar for Logic systems). Low pressure can cause circulation/temperature issues; repressurise following the user manual if needed.

3) Ensure all radiator and isolating valves are open so the system can circulate. Bleed radiators if needed to remove airlocks that may affect temperatures.

4) Note whether domestic hot water still works (on system boilers DHW is cylinder-heated and may behave differently). Record any other error codes or symptoms (e.g., L1 overheat, pump noise, low pressure).

Specific diagnostic steps (if you are competent with basic electrical isolation and comfortable removing the front panel):

1) Isolate electrical supply at the fused spur and wait for the boiler to cool. Remove the front cover per the manufacturer instructions. Only proceed if you are comfortable with this and have isolated power.

2) Locate the return thermistor: it is usually an NTC sensor clipped to the return pipework near the heat exchanger or pump and wired back to the PCB. Verify the sensor is physically clipped to the pipe (good thermal contact), not loose or switched with the flow thermistor.

3) Visually inspect the thermistor wiring and connector: look for loose plugs, corroded pins, frayed or damaged cable and any signs of water ingress. Reseat the sensor connector on the PCB (push it out and back in firmly) and ensure the clip fits tightly to the pipe.

4) If you have a multimeter and know how to use it safely, you can measure the thermistor resistance at room temperature and while the system is cold/hot and compare to expected NTC curves (typical NTC behaviour is decreasing resistance as temperature rises). If you do not know expected values, do not guess—this test is best left to an engineer.

5) If the sensor or wiring appears damaged, replacing the return thermistor typically fixes the fault. This replacement requires accessing and changing parts inside the boiler and should be done by a Gas Safe engineer unless you are qualified.

6) If the sensor and wiring appear OK but the fault persists, the issue may be a faulty PCB or incorrect sensor-to-board communication (or a large flow/return temperature differential). These faults require diagnostic equipment and component-level work by a qualified technician.

When to call a professional:

- If the fault reappears after a reset, if you find damaged wiring, or if you are not confident doing the safe-isolate and cover removal, call a Gas Safe registered heating engineer.

- Any replacement of sensors, access to the PCB, gas-side work, or detailed electrical diagnostics must be carried out by a qualified engineer.

Why call a pro and what to tell them:

- Explain you have an Ideal Logic/Logic Plus showing L5 or F5 (return thermistor fault). Tell them what checks you have already done: reset, pressure check, bleeding radiators, whether DHW still works, and any visible wiring damage or loose connectors you found. This helps the engineer diagnose faster.

Final note: a temporary reset may get the boiler back into service, but persistent L5/F5 indicates a sensor or control fault that should be repaired by a Gas Safe registered engineer to ensure safe, reliable operation.