What the F4 code means
A thermistor is a small temperature sensor whose electrical resistance changes as it heats up and cools down. The flow thermistor sits on the pipework at the top of the heat exchanger, where hot water leaves the boiler on its way to your radiators or hot water. The control board reads it constantly to decide how hard to fire the burner and when to turn it down.
If the reading is missing, or falls outside the range the board expects, the boiler has no reliable way of knowing how hot the water is. Carrying on could mean overheating, so it shuts down and shows F4. On Ideal's Logic Code Combi, Logic+ Combi and Logic Combi2 manuals the fault is listed as F4 or L4, flow thermistor fault. The F version can clear once the reading returns to normal, while L4 is a lockout that needs a reset.
Code lists do vary between Ideal ranges, and a few older or specialist models use different numbering, so check the fault table in your own manual.
Common causes of F4
A failed thermistor
Thermistors are simple parts, but they do fail with age and heat. When one goes open circuit or short circuit, the board sees an impossible reading straight away.
Poor connections or damaged wiring
The thermistor is connected to the main board by a thin wiring loom. A loose plug, a corroded terminal or a wire damaged by heat can cause a reading that jumps around. This often shows as an F4 that comes and goes.
Water getting onto the sensor
A small internal leak or drips from a joint above the sensor can affect the connection and trigger the fault. If you have seen water under the boiler, mention it to the engineer. Our guide on a boiler leaking water covers the warning signs.
A control board fault
If the thermistor and its wiring test correctly, the fault may lie with the circuit on the board that reads it. This is less common and is usually the last thing an engineer checks.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Failed flow thermistor | F4 or L4 returns straight after a reset | Gas Safe engineer |
| Loose or corroded connection | Fault comes and goes | Gas Safe engineer |
| Water on the sensor or wiring | Drips or damp under the boiler | Gas Safe engineer |
| Control board fault | Sensor tests fine but code persists | Gas Safe engineer |
Safe checks you can do yourself
A thermistor fault cannot be repaired from outside the boiler, but these checks are worth making before you book an engineer.
- Check the power supply. Make sure the fused spur is on and there has been no recent power cut or tripped circuit.
- Restart the boiler. Ideal suggests a restart as the first step. Press reset as your manual describes, or switch off at the fused spur for a minute and switch back on.
- Watch what happens. Note whether the code comes back straight away, after a few minutes of heating, or only occasionally. This pattern helps the engineer.
- Check the pressure gauge. It should read 1 to 1.5 bar when cold. Low pressure will not cause F4, but it is worth knowing if it is also low. See our guide on repressurising a boiler.
- Look under the boiler for water. Do not open the casing, but note any drips or damp patches on the pipes underneath.
- Stop after two or three attempts. If the fault keeps returning, call an engineer.
Do not remove the casing to reach the thermistor. The sensor sits next to hot pipework and live electrical connections inside the boiler, and all internal work must be done by a Gas Safe registered engineer.
When to call a Gas Safe engineer
Call a Gas Safe registered engineer if F4 or L4 comes back after two or three restarts, or if it keeps appearing every few days. The engineer will measure the thermistor's resistance at a known temperature, which quickly shows whether the part has failed. They will also check the plug, wiring and board connections.
The good news is that a flow thermistor is one of the cheaper parts on a boiler, and many engineers carry them for common Ideal models. When you book, give the code, the model name and whether the fault is constant or intermittent. If you have seen other codes recently, such as an L1 overheat lockout, mention those too, as they can point to a sensor that has been giving poor readings for a while.
Typical repair costs in the UK
These are typical ranges including parts and labour. Your final price will depend on the model, your area and the engineer.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Flow thermistor replacement, fitted | Typically £90 to £180 |
| Repairing a loose or corroded connection | Typically £80 to £150 |
| Fixing a small internal leak | Typically £100 to £250 |
| Replacement PCB | Typically £300 to £550 |
How to stop F4 coming back
Thermistors wear out in their own time, so there is no way to guarantee one will last. What you can do is keep the boiler serviced every year, as an engineer can spot corroded connections, weeping joints and heat-damaged wiring before they cause a breakdown. Our boiler service cost guide explains what a service covers. Deal with any leak under the boiler quickly, as water near electrical parts causes a range of faults. If your boiler shows a related sensor code for the return pipe instead, see our page on the Ideal F5 return thermistor fault.