Overview
F5 on an Ideal boiler means the control unit has detected a fault with the return thermistor (the temperature sensor on the return pipe). The return thermistor tells the boiler the temperature of water coming back from the heating system; if its signal is missing, out of range or inconsistent the boiler will show F5 and may lock out or suspend central heating until the fault is resolved. Common causes are a failed NTC thermistor, a damaged or disconnected sensor cable/plug, a poor sensor clamp/position on the return pipe, circulation problems that create an abnormal temperature difference (airlocks, seized pump or closed valves), or rarely a PCB/wiring fault. Severity is moderate: it usually does not present an immediate safety risk like a gas leak, but it will prevent normal heating operation and should be fixed promptly. Some simple checks (restarting the boiler, topping up pressure, bleeding radiators) can be done by a homeowner, but diagnosing and replacing sensors, fixing wiring or addressing pump/PCB faults should be left to a Gas Safe qualified engineer.
Troubleshooting Steps
Safety precautions:
- If you are not competent with gas and mains electrical appliances do not open the boiler or work on gas or live wiring. Only a Gas Safe registered engineer should work on gas components or replace sealed parts.
- Before doing any checks that involve the boiler internals, switch off the electrical supply at the isolated fused spur and isolate the gas if advised by the manufacturer’s manual.
- Take care with hot surfaces and hot water when bleeding radiators or touching pipework.
Initial checks a homeowner can do:
1) Reset the boiler: try the boiler reset or switch power off at the fused spur for 30 seconds then back on. Note if the F5 returns immediately or after a short run.
2) Check pressure: confirm system pressure is within the normal range (usually about 1.0–1.5 bar cold on most boilers). If low, repressurise following the boiler manual and then reset.
3) Bleed radiators: airlocks can cause abnormal temperatures. Bleed radiators to remove trapped air and then top up to correct pressure.
4) Check visible wiring and connectors: with the boiler OFF, look at the external wiring to the boiler and the plug-in connectors for any obvious damage, corrosion or loose plugs. Do not probe live terminals.
5) Listen for the pump: when there is a CH demand, you should hear the pump running. If the pump is silent or rattling, note that for the engineer.
Specific diagnostic and fix steps (for a competent DIYer only):
1) Check thermistor clamp position: the return thermistor must be solidly clipped to the return pipe. If it has slipped or is not clamped to the pipe, reposition and secure it, then reset the boiler and test. Poor contact can give erroneous readings.
2) Inspect the sensor and connector: with power isolated, unplug the return thermistor connector at the wiring loom and inspect for corrosion, bent pins or water ingress. Re-seat clean connectors. If the connector looks damaged, do not force repairs; call an engineer.
3) Measure sensor resistance (only if comfortable with multimeter and with power isolated): remove the thermistor lead and measure resistance across the sensor. A healthy thermistor gives a temperature-dependent resistance (not open circuit). If it reads open circuit or wildly different to the flow thermistor under the same conditions, the return thermistor is likely faulty and should be replaced. Consult the boiler documentation for correct sensor type/spec or have the engineer confirm.
4) Check continuity back to the PCB: if the sensor looks OK, check continuity of the cable from the sensor plug to the PCB connector (with power isolated). A break indicates cable replacement/repair is needed.
5) Check for circulation issues: if the sensors and wiring are intact, circulation faults (seized pump, closed radiator valves, faulty motorised valve or airlock) can cause an abnormal flow/return temperature differential and trigger F5. Bleeding radiators, ensuring all radiator and system valves are open, and confirming the pump runs are valid checks. If the pump is noisy, not running or you suspect a seized rotor, stop and call an engineer.
6) Reset and test: after any corrective action, restore power, reset the boiler and run a heating demand. If the F5 persists, record when it appears (immediately, after pump runs, after a few minutes) and any other symptoms.
When to call a professional:
- If you find a failed thermistor, damaged wiring, or water damage to connectors you should call a Gas Safe registered heating engineer to replace the sensor and carry out any necessary repairs.
- If the F5 persists after simple homeowner checks (repressurising, bleeding, reseating sensor clamp) or if you suspect pump failure, motorised valve problems or a PCB fault, do not attempt further internal repairs yourself.
- Always call a Gas Safe engineer for any work that involves gas, sealed components, the PCB, or replacement of sensors inside the boiler casing.
Notes and safe practice:
- Do not repeatedly reset the boiler to clear the error; if the fault reappears call an engineer.
- If your boiler is under warranty or service plan, contact the manufacturer or your service provider before attempting internal repairs.
- When contacting the engineer, tell them the F5 code, what checks you have already done (pressure, bleeding, pump sound, connector condition), and whether the fault is constant or intermittent. This speeds diagnosis and repair.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Ideal Ideal Boiler.