Overview
This fault indicates the boiler is seeing an abnormal or missing temperature reading from the return-side thermistor (the NTC sensor that measures the water temperature returning to the boiler). The boiler uses that return temperature to control combustion and condensate operations; if the measured value is out of expected range or absent it will lock out and show F5 (or on some Logic models L5). The appliance treats this as a safety/control fault rather than an immediate gas leak or fire hazard, but it will often prevent the boiler from operating until the condition is resolved. Common reasons are a failed (open or shorted) return thermistor, a loose/damaged sensor cable or connector, the sensor not clipped correctly to the return pipe (or fitted to the wrong pipe), or a circulation problem causing an unexpected temperature difference (airlock, seized/isolated pump, closed valves, low system water pressure). Less commonly the boiler PCB or wiring to the PCB has failed, or recent servicing/installation left the sensor disconnected or mis-routed. Some simple checks and resets can be done by a homeowner, but diagnosing/fixing sensor replacement, wiring or PCB faults and any work involving gas or exposed live parts should be carried out by a Gas Safe registered engineer.
Troubleshooting Steps
Safety precautions:
1. If you smell gas or suspect a gas leak, do not touch the boiler — leave the premises and call your gas emergency number immediately.
2. Before opening the boiler casing or touching internal components, isolate electrical power at the boiler fuse spur and switch off the boiler. If you are not comfortable or confident working near mains electricity, do not open the casing.
3. Do not attempt to work on gas valves, burners, or the PCB. Any internal repairs or sensor replacements should be done by a Gas Safe registered engineer.
Initial homeowner checks (safe, basic steps):
1. Note the exact fault code and when it appears. If L5/F5 is displayed, try a simple reset by switching the boiler off at the fuse spur or isolator for 10–30 seconds, then switch back on. Record whether the fault returns and how quickly.
2. Check the system water pressure on the boiler gauge. For Logic/Logic Max system boilers you should normally see about 1.0–1.5 bar (follow manufacturer guidance). If pressure is low, top up the system using the filling loop until the pressure is between 1.0 and 1.5 bar, then press RESTART. If you do not know how to repressurise, consult the manual or your installer.
3. Bleed radiators to remove airlocks and help circulation. After bleeding, check pressure and repressurise if needed. Reset the boiler and check if the code clears.
4. Listen for the pump when the boiler calls for heating. Feel radiators: if the top is cold and bottom hot (or no heat at all), circulation may be poor. Make sure radiator thermostatic valves and boiler/zone valves are open.
Specific diagnostic steps you can try (visual, non-invasive):
1. With the boiler powered down at the fuse spur, remove the outer casing only if you are comfortable doing so and the manual allows it. Inspect the return thermistor cable running to the PCB and the sensor clip on the return pipe. Look for obvious damage, loose connectors, or a sensor that is not clipped securely to the copper pipe. Some sensors are small black/grey probes clipped to the pipe — they must be firmly in contact with the pipe.
2. If the sensor is visibly loose or detached from the pipe, and you are confident and safe to do so, re-seat the clip so it sits firmly on the correct return pipe. Replace the casing, restore power and restart the boiler. If the fault clears, monitor the boiler for recurrence.
3. If wiring at the sensor or the terminal block looks frayed or loose and you understand basic electrical safety, you can visually check that connectors are seated. Do not strip wires, solder, or connect while power is live. If anything looks damaged, stop and call a Gas Safe engineer.
4. Check whether the flow and return thermistors have been swapped or fitted to the wrong pipes (this can happen after maintenance). If the readings are obviously reversed (flow much colder than return) this will cause faults and needs an engineer to correct.
5. If you get a return thermistor fault together with a large flow/return temperature differential (for example a differential greater than about 40–50°C), suspect circulation restriction. Check that pump isolator valves are open, that the pump runs, and that magnetic debris or sludge hasn’t blocked the pump or system. Bleeding and flushing the system or fitting a magnetic filter are engineer tasks.
When to call a professional and what to tell them:
1. Call a Gas Safe registered heating engineer (or Ideal Heating if under warranty) if the code persists after resets, after repressurising and bleeding, or if any sensor wiring or the PCB looks damaged. Also call an engineer if the error appears intermittently or under load, or if you are unsure about any step.
2. When you call, tell them: model (Ideal Logic Max / Logic Plus), exact fault code (L5 or F5), what you have already tried (reset, pressure level, bleeding radiators, pump noises), system pressure reading, and whether any recent work or power cuts occurred.
3. The engineer will typically test sensor resistance values, check wiring and terminal connections, confirm the sensor is fitted to the correct pipe, and may replace the return thermistor if faulty. If the sensor and wiring are OK they will investigate circulation (pump, valves, airlocks, blockages) and, if needed, PCB faults.
Final notes:
- Small, safe steps (reset, check pressure, bleed radiators, re-seat sensor clip) are reasonable for a competent homeowner. Avoid attempting electrical or gas repairs yourself.
- If the boiler is under warranty, contact the manufacturer or the installer before opening the casing or replacing parts, as DIY intervention can affect cover.
- Persistent F5/L5 faults commonly mean a failed thermistor, a loose/damaged connection, or a circulation issue; a qualified engineer is usually needed to complete a safe, correct repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Ideal Logic Max & Logic Plus Regular Boiler.