Overview
F6 on Ideal Logic Max / Logic Plus regular boilers indicates an outside (external) temperature sensor fault. This sensor is used for weather compensation or to help the boiler modulate output based on external temperature. The fault means the boiler is not getting a valid temperature reading from that sensor — caused by a failed thermistor/sensor, broken/damaged wiring, a loose or corroded connector, water ingress or frost damage to the probe, or less commonly a fault on the boiler PCB or sensor input circuit. In many installations the boiler will continue to provide hot water and heating using internal controls, but the absence of the outside sensor can cause incorrect modulation and inefficiencies and in some cases the boiler may log the fault and refuse to operate until cleared. Severity is generally moderate: it is not normally a direct gas safety emergency, but it can lead to incorrect boiler behaviour (overheating, poor comfort control, or repeated lockouts). Basic checks and a reset can sometimes clear a transient fault, but persistent F6 faults normally require a competent heating engineer to test and replace the sensor or repair wiring. Do not attempt gas or mains-electrical repairs unless you are a qualified engineer; simple visual checks and a power reset are reasonable for a homeowner to try first.
Troubleshooting Steps
Safety precautions:
- If you are not confident working around electrical wiring or boiler internals, stop and call a Gas Safe registered engineer. Incorrect work can be dangerous and may invalidate warranties.
- Before touching any connectors inside the boiler, switch the boiler off at the programmer, then isolate electrical power at the boiler fuse spur or the mains isolator. Wait a few minutes to let electronics discharge.
- Do not tamper with gas valves, burners, or sealed combustion parts. If you smell gas, leave the property and call the gas emergency number immediately.
Initial checks a homeowner can do:
1. Note the exact error (F6), any other codes, and when it started (after power cut, storm, freezing weather, recent works). Take a photo of the display for the engineer if needed.
2. Try a simple reset: use the boiler RESET/RESTART button or switch the boiler off at the mains spur for 30 seconds and turn it back on. Check if the fault returns.
3. Visually inspect the outside sensor (if accessible): many are mounted on an external wall in a small housing or clipped to pipework. Look for obvious damage, water ingress, frost or physical disconnection, crushed or chewed cable.
4. If the sensor is frozen or covered in ice, gently thaw it (warm water, hairdryer at a distance or bring it inside briefly) and then reset the boiler. Do not apply naked flames or overheat the probe.
Specific diagnostic and fix steps (for competent DIYers only):
1. With the boiler electrically isolated, open the boiler cover and locate the outside sensor wiring terminal (refer to your manual for terminal identification). Check the connector is tight and corrosion-free. Re-seat the connector if it looks loose.
2. Inspect the sensor cable for cuts, crushed sections, or rodent damage along its visible route. Repairing or replacing cable should only be done if you are competent with low-voltage wiring; otherwise call an engineer.
3. If you have basic electrical test tools and know how to use them safely: with power off you can check for continuity in the sensor cable and a likely resistance value at room temperature (typical NTC thermistors are around 10kΩ at 25°C but consult the boiler/manual for exact figures). If the sensor is open-circuit or the resistance is wildly out of range, the sensor is faulty and needs replacement.
4. If the sensor looks fine and wiring/connector checks out, restore power and monitor the boiler display. If the fault persists immediately, do not continue trying to operate the boiler; record the failure behaviour for the engineer.
When to call a professional and what to tell them:
- If the F6 fault returns after a reset, you cannot locate or safely access the sensor/cable, the wiring shows damage, or you do not have the skills to test component resistance, call a Gas Safe registered heating engineer.
- Ask the engineer to check the outside sensor probe, cable continuity, connector, and the sensor input on the PCB. Mention any recent external work, freezes, or pest activity that might explain cable damage.
- Be ready to provide the boiler model, serial number, when the fault started, and photos of the sensor and any damaged cable; this helps diagnosis and allows them to bring the right replacement parts.
Final note:
Do not attempt internal PCB repairs or gas/combustion component work yourself. Persistent F6 faults are commonly resolved by replacing the outside sensor or repairing its wiring, but diagnosing the root cause and fitting parts should be done by a qualified engineer to ensure safe and correct repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Ideal Logic Max & Logic Plus Regular Boiler.