Overview
F4 on an Ideal boiler indicates a fault with the flow thermistor — the temperature sensor that monitors the water leaving the boiler. The boiler uses that sensor to control burner operation and pump behaviour; if the sensor reading is missing, incorrect or out of range the boiler will often lock out or run poorly to protect itself. The error can be caused by a failed thermistor, poor or corroded wiring/connectors, incorrect system pressure, or circulation problems (pump failure, air in the system, blocked pipework or valves). Severity is moderate: the boiler will usually go into lockout or stop providing heating/hot water until the issue is resolved, so you may be without heating and hot water. It is not usually an immediate safety danger because the boiler’s safety logic prevents unsafe operation, but you should not ignore it because running without proper temperature feedback can lead to further component stress. Some basic checks and a reset are safe for a competent homeowner, but replacing sensors, working inside the casing, or any work affecting gas or sealed hydraulic components should be done by a Gas Safe registered engineer.
Troubleshooting Steps
Safety precautions:
- Before doing anything, switch the boiler to standby and isolate electrical power at the mains or the boiler isolator. If you need to work on gas or internal components, stop and call a Gas Safe engineer. Never attempt to repair gas valves or internal gas parts yourself.
- Allow the boiler to cool before touching pipework or components to avoid burns.
Initial checks a homeowner can do:
1. Note any accompanying behavior: is there no heating, no hot water, or is the boiler intermittently working? Take a photo of the display if helpful.
2. Check the boiler pressure gauge. Ideal boilers normally should sit around 1.0–1.5 bar cold (check your manual). If pressure is low (below about 0.7–0.9 bar) this can cause faults. If pressure is high (over 2.5 bar) that is also problematic.
3. Try a simple reset: switch the boiler off at the front panel or isolator, wait 2–5 minutes and switch it back on, then attempt to operate heating/hot water again. Some transient errors clear with a reset.
4. Look for obvious leaks around the boiler and pipework and check radiators for cold spots or air (which suggest circulation issues).
5. Bleed radiators that have cold tops to remove airlocks. This can restore circulation without opening the boiler.
Specific diagnostic and basic fix steps (homeowner scope):
1. If pressure is low and you are comfortable doing so, repressurise the system using the boiler’s filling loop per the boiler manual: switch the boiler off, open the filling loop valves slowly while watching the pressure gauge until it reaches roughly 1 to 1.5 bar, then close the valves and restart the boiler. If pressure drops again quickly, you have a leak and must call an engineer.
2. After topping up pressure and bleeding radiators, reset the boiler again. If F4 clears and the system runs normally, monitor pressure over the next 24–48 hours.
3. If the boiler is making unusual sounds or the pump doesn’t appear to run (pipes stay cold and radiators stay cold despite a demand), do not try to dismantle the pump. Record symptoms and call an engineer.
When to stop and call a professional:
- If F4 persists after a reset, after correct system pressure and after bleeding radiators, stop further DIY. The likely causes from this point are a failed flow thermistor, damaged wiring or connector, or an internal circulation fault. Diagnosing or replacing the thermistor requires access inside the boiler and electrical testing and therefore must be done by a Gas Safe registered engineer.
- If you find evidence of a leak, repeated pressure loss, or if the boiler displays additional fault codes, call a Gas Safe engineer.
- If you are not confident using a multimeter or working with the boiler electrical isolation, do not proceed; call a professional.
Notes on professional diagnosis and repair:
- A qualified engineer will isolate the unit, remove the casing, check the flow thermistor wiring and connector, measure the sensor resistance/behaviour with the boiler running, and verify pump operation and circulation. They will replace the thermistor and/or rectify wiring, fix any circulation blockages, or repair other faults as found. They will also check the PCB and software if required and re-run the system to confirm the fault is cleared.
- Keep the boiler manual, the serial/model number, and a note of when the fault started available for the engineer. Persisting F4 faults should be handled by a Gas Safe registered heating engineer — do not attempt gas or sealed hydraulic repairs yourself.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Ideal Ideal Boiler.