Ideal F4 fault code: what does it mean and can you fix it?

What does the Ideal F4 fault code mean?

Ideal F4 means your boiler cannot get a sensible reading from its flow thermistor, the sensor that measures how hot the water is as it leaves the heat exchanger. Without a trustworthy reading it cannot control its own temperature safely, so it stops. You cannot fix this yourself. It needs a Gas Safe registered engineer.

Ideal's own consumer fault-code page does list F4, but it gives you two words and nothing else: "Flow Thermistor". No explanation, no causes, no next step, so here is the rest of it.

The installation and servicing manuals are more useful, and they say the same thing in two different ways. The Ideal Logic Code Combi ESP1 Installation & Servicing manual heads the section "'F4 Or L4' - Flow Thermistor Fault". The Ideal Logic Max Heat Installation & Servicing manual's fault-finding chart writes the same code as "'L4' or 'F4' - CONTROL/NO FLOW THERMISTOR FAULT". Ideal's wording is not consistent between ranges, so we have quoted both rather than pick one.

Note the "F4 or L4" in both. Several Ideal ranges display this identical fault as L4. If your screen shows L4, read this page as if it said L4 throughout, because it is the same sensor and the same repair.

One thing worth saying plainly: F4 points at the sensor and its wiring, not at your heating water, so topping the pressure up is not the fix for F4 itself. Read the gauge and note the figure for the engineer. In some cases poor circulation makes a perfectly healthy sensor report a real but abnormal temperature, and we cover that further down. None of the Ideal documents we checked describe F4 as a gas-safety code. If you ever smell gas or your carbon monoxide alarm sounds, that is a separate emergency whatever is on the screen. Call the National Gas Emergency Service on 0800 111 999 straight away.

Key facts

The short version, before the detail.

• What it means: the flow thermistor, the sensor reading the temperature of water leaving the heat exchanger, is not giving the boiler a reading it can trust, so the boiler stops.
• Can you fix it yourself? No. Every real fix for F4 is engineer work, and most of it sits behind the boiler casing: the sensor, its wiring and the circuit board. Where poor circulation is behind the code, the work moves out to the pump, valves and system filter, still an engineer job. One reset, per your Ideal user guide, is the only thing worth trying.
• It is an engineer job from the moment F4 comes back after that single reset. The sensor, its wiring and the circuit board are all Gas Safe registered engineer work.
• The same fault shows as L4 on several Ideal ranges, and Ideal's own manuals head the section "F4 or L4".
• Models affected: Ideal Logic, Logic+, Logic Max, Logic Heat and Heat2, Logic Code Combi ESP1 (26/33/38), Logic Plus Combi (C24/C30/C35), Vogue, Vogue Max and Vogue Gen 2. Ideal's fault-code page presents F4 as a general Ideal code rather than restricting it to one range.
• The number an engineer works to: Ideal's manual expects the thermistor to read roughly 9,700 to 10,300 ohms at 25°C, 2,400 to 2,600 ohms at 60°C and 1,000 to 1,100 ohms at 85°C. A reading outside those bands points at the sensor itself.
• What it is likely to cost: the average UK boiler repair is around £300, with a usual range of £120 to £750, and a replacement circuit board runs £250 to £450 (Checkatrade 2026 cost guide).
• Cover is for future breakdowns. An F4 showing on your boiler today counts as a pre-existing fault and is not covered, so book a one-off repair for that.
• Smart Plan boiler cover is a service plan, not insurance. Parts and labour are included up to £500 a year if the boiler is under 7 years old, or up to £200 a year if it is over 7. Those are caps, not a promise the whole repair is paid for, and a bigger job can cost more, with the difference down to you.
• Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, with an engineer attending outside those hours only for a genuine emergency breakdown. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, if the engineer cannot get access, and for early annual-service requests.
• Using a module starts a 12-month agreement period from the first time you use a service. There is a 14-day cooling-off period, which ends the moment a service is carried out, and leaving early after that costs the remaining months or 75% of the outstanding balance.

Why does my Ideal boiler keep showing F4?

F4 is a sensor fault, so the cause sits in one of three places: the flow thermistor itself, the wiring between it and the boiler's main circuit board, or the board. There is a fourth possibility worth knowing about, because it changes the repair completely. Sometimes the sensor is working perfectly and is reporting a real but abnormal temperature caused by poor water circulation, in which case replacing the sensor fixes nothing.

An engineer separates these by measuring the thermistor's resistance against Ideal's published figures. The Logic Code Combi ESP1 manual gives roughly 9,700 to 10,300 ohms at 25°C, 2,400 to 2,600 ohms at 60°C and 1,000 to 1,100 ohms at 85°C. A reading outside those bands points at the sensor. A reading inside them means the sensor itself is not the fault, and the engineer then has to rule out the wiring, the low-voltage connector on the circuit board, the board, poor circulation, and a sensor that has come off its pipe clip. One of the checks Ideal's own manual sets out is whether the wiring is securely connected to the flow thermistor in the top of the heat exchanger.

Ideal F4 causes and who fixes them

Can I fix an Ideal F4 fault myself?

Honestly, no. Every genuine fix for F4 is engineer work, and most of it is behind the boiler casing: testing a sensor, reseating a plug, tracing a wire, changing a circuit board. Where poor circulation turns out to be the cause, the job moves out to the pump, the valves and the system filter, which is still an engineer's work. We are not going to pretend otherwise to make this page longer.

What you can do is one reset and a handful of checks that either rule out something simpler or save the engineer time on the day. None of them involve opening the boiler. Work through them in this order, then stop.

1. Read the pressure gauge on the front of the boiler and note the reading. It does not fix F4, but the engineer will ask you for it.

2. Check your room thermostat is calling for heat and its batteries are not flat, so you can rule out a control problem before the visit.

3. Look at the outside of the boiler and the pipes around it for obvious damage or water leaks. Do not open the casing or remove any panel.

4. Check whether your other gas appliances, such as a gas hob or fire, still work, so you know the gas supply itself is on. Never use a gas hob, oven or an unflued heater to warm a room while you wait for an engineer, because that is a carbon monoxide risk. Use electric heating instead.

5. Press the reset button once only, exactly as your Ideal user guide describes. If F4 comes straight back, stop and book a Gas Safe registered engineer. Do not keep resetting.

6. Book a Gas Safe registered engineer. F4 is a sensor fault inside the boiler and there is nothing further a householder can safely do.

That is genuinely the end of the safe list. If F4 returned after one reset, repeating it will not help. You would only be clearing the display, not the fault, and a boiler that keeps locking out is telling you something real.

Never open the casing, and never touch the thermistor, its wiring or the circuit board. If you smell gas or your carbon monoxide alarm sounds at any point, treat that as a separate emergency. Call the National Gas Emergency Service on 0800 111 999 immediately and follow the instructions they give you. Open doors and windows, leave electrical switches alone, and get everyone out of the house.

Is F4 the same as L4, F5 or F0?

These codes get mixed up constantly, and two of them genuinely are the same fault. Here is how they separate.

L4 is the same flow thermistor fault. Several Ideal ranges display it as L4 rather than F4, and Ideal's own manuals head the section "F4 or L4", with the Logic Max Heat chart writing it as "'L4' or 'F4'". If your boiler shows L4, everything on this page applies to you.

F5 is a Return Thermistor fault, listed as "Return Thermistor" on Ideal's fault-code page. It is the matching sensor on the pipe carrying water back into the boiler. It is easily confused with F4 because the test values and the repair are almost identical, but it is a different sensor in a different place.

F0 is "No Water Flow Thermistor" on Ideal's own page. It is closely related to F4, and confusingly some Logic Max manuals write the F4 text itself as "CONTROL/NO FLOW THERMISTOR FAULT", which is exactly why the two get muddled.

F3 is a Fan Fault, a problem with the combustion fan rather than a temperature sensor. It is nothing to do with F4 beyond the digits sitting next to each other, which is why F3 gets mistyped as F4.

F6 is an Outside Sensor Fault, the external weather-compensation sensor. That one is outside the boiler entirely.

L1 is a Flow Temperature Overheat or No Water Flow lockout. Its symptoms overlap with F4 because poor circulation can trigger either, but L1 is a lockout on actual overheating rather than a sensor the boiler cannot read.

F1 is Low Pressure, the most common Ideal code by far and the one people often expect when any F code appears. It has nothing to do with the flow thermistor.

One last check worth making: Ideal writes its codes without a decimal point, so a code shown on your screen as "F.4" is unlikely to be an Ideal code at all. A code written in that dotted style, such as F.4 or F.75, is Vaillant's format, a different manufacturer with a completely different code table. Check the badge on your boiler before following any code guide, because reading the wrong maker's page will send you the wrong way.

When should I call a Gas Safe registered engineer?

As soon as F4 has come back after a single reset. There is no waiting game with this one, because there is nothing left on your side of the casing to try.

It helps to know what the visit actually looks like, so you can tell whether the work is being done properly. An engineer will take the casing off, find the flow thermistor in the top of the heat exchanger, and check the wiring is securely connected there, which is one of the checks Ideal's own manual sets out. They will then measure the sensor's resistance against Ideal's figures: roughly 9,700 to 10,300 ohms at 25°C, 2,400 to 2,600 ohms at 60°C, 1,000 to 1,100 ohms at 85°C. If it reads outside that, the sensor is replaced. If it reads inside those bands, the sensor itself is not the fault, and the engineer has to rule out the wiring loom, the low-voltage connector on the main circuit board, the board, poor circulation, and a sensor that has come off its pipe clip.

The one thing worth asking them directly: if the sensor tests fine, is circulation the problem? A stuck pump, a closed valve, a blocked filter or sludge in the heat exchanger can all make a healthy sensor report an abnormal temperature. Changing the sensor in that case costs you a part and fixes nothing. It is a fair question to ask before any part is ordered.

On cost, the average UK boiler repair is around £300, with a usual range of £120 to £750 depending on the fault and where you live, and a replacement circuit board runs £250 to £450 (Checkatrade 2026 cost guide). A thermistor sits at the cheaper end, a board at the dearer end. You can check any engineer's registration on the Gas Safe Register before they start. Ask them to confirm which part they have diagnosed, and to price it, before the work goes ahead.

If you would rather not face the bill cold next time, Smart Plan boiler cover is worth a look, with one thing said up front: cover is for future breakdowns, so an F4 showing on your boiler today counts as a pre-existing fault and is not covered. Book a one-off repair for that. Smart Plan is a service plan, not insurance. It is modular, so you build your own cover and pick only the parts of your home you want looked after. Boiler and central heating cover includes parts and labour up to £500 a year if your boiler is under 7 years old, or up to £200 a year if it is over 7. Those are caps, not a promise the whole repair is paid for: on a boiler over 7 the limit is £200 a year, and a job like a new circuit board can cost more than that, with the difference down to you. We work with a national network of thousands of Gas Safe registered engineers, and we aim to get an engineer out to you within two working days.

A few things worth knowing before you join, so there are no surprises. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, with an engineer attending outside those hours only for a genuine emergency breakdown, so do not rely on cover for evening or weekend help with a routine F4. A £95 call-out fee, paid in advance, applies in your first 30 days, for faults the plan does not cover, if the engineer cannot get access, and for early annual-service requests. Using a module starts a 12-month agreement period from the first time you use a service. There is a 14-day cooling-off period, and it ends the moment a service is carried out. Leaving early after that costs the remaining months or 75% of the outstanding balance.

Would rather just get it fixed now? Book a one-off repair and a Gas Safe registered engineer will come out to you. One-off repair visits run Monday to Friday, 09:00 to 17:00, and are not covered by a workmanship guarantee. Call us on 0333 772 6247, Monday to Friday, 08:00 to 18:00, and we will take it from there.

Ideal F4 fault code FAQs

Seeing F4 or L4 on your Ideal boiler? We can get an engineer out to you.

Book a one-off repair, Monday to Friday, 09:00 to 17:00, with no workmanship guarantee, and a Gas Safe registered engineer will test the flow thermistor, its wiring and the board to identify the fault. Or set up a Smart Plan boiler module for future breakdowns, remembering it will not cover an F4 that is already showing today. A service plan, not insurance: parts and labour up to your cover limit, with cover-plan call-outs Monday to Friday, 08:00 to 18:00, and an engineer attending outside those hours only for a genuine emergency breakdown. A £95 call-out fee, paid in advance, applies in set cases including your first 30 days, and using a service starts a 12-month agreement period, with early exit costing the remaining months or 75% of the outstanding balance.

Helpful links

• Build your boiler cover
• All Ideal fault codes
• Ideal L5 fault code (reset lockout or return thermistor, depending on model)
• Ideal L1 fault code (flow temperature overheat or no water flow)
• Ideal F1 fault code (low pressure)
• Ideal F2 fault code
• Ideal Heating help hub
• Boiler repair
• Boiler & central heating cover
• UK boiler repair costs
• Ideal's official boiler fault-code list
• Check an engineer on the Gas Safe Register