Ideal F5 fault code: what does it mean and can I fix it?

What does the Ideal F5 fault code mean?

Ideal F5 means the return thermistor has stopped giving a sensible reading. That is the small temperature sensor clipped to your central heating return pipe, which tells the boiler how hot the water is when it comes back from the radiators. You cannot fix it at home. Reset once, and if F5 returns, book a Gas Safe registered engineer.

Ideal publishes this one on its own consumer support site. Its official fault-code page states: "F5 - Return Thermistor. Reset the boiler. If this does not fix the problem, please contact us quoting error code F5 as this will require a visit from an engineer." Ideal's Logic Code Combi ESP1 installation and servicing manual, the engineer-facing document, heads the same fault "F5 or L5 - Return Thermistor Fault", and the Vogue Max System manual calls it a Return Thermistor Fault.

Without that return reading the boiler cannot judge how hard to fire, so it stops rather than run blind. That is the boiler protecting itself, and it is a sensor or wiring fault rather than a gas escape. It still needs a Gas Safe registered engineer, because every part of the repair sits behind the sealed casing.

One thing to be clear about before you read on. The causes, the resistance figures and the diagnostic order further down this page come from Ideal's installation and servicing manuals, which are written for engineers. They are here so you know what is going on and what the visit will involve, not so you can work through them yourself.

Ideal does not describe F5 as a gas-safety code: its own documents treat it as a sensor or wiring fault. If you ever smell gas or your carbon monoxide alarm sounds, that is a separate emergency whatever is on the display. Call the National Gas Emergency Service on 0800 111 999 straight away.

Key facts

The short version, if you are standing in front of the boiler right now.

• What it means: the return thermistor, the temperature sensor on your central heating return pipe, has stopped giving the boiler a usable reading, so the boiler shuts down rather than fire blind.
• Ideal's own words: its public fault-code page lists "F5 - Return Thermistor" and tells you to reset the boiler, then contact an engineer quoting F5 if that does not clear it. Ideal's Logic Code Combi ESP1 installation and servicing manual calls the same fault "F5 or L5 - Return Thermistor Fault".
• DIY-fixable? No. A single reset is the only fix you can attempt at home. Every repair for F5 sits behind the boiler casing.
• It becomes an engineer job the moment the code returns after that one reset. Testing, rewiring or replacing the thermistor, and replacing the PCB, are all Gas Safe registered engineer work.
• Models: Ideal's current condensing ranges that use the F and L code set, including Logic Combi, System and Heat, Logic+, Logic Max, Logic Heat2, Logic Code Combi ESP1, Vogue, Vogue Gen2 and Vogue Max. Older ranges such as Isar, Icos and Mexico HE publish their own separate code lists, so check that model's guide.
• The figures the engineer checks: Ideal's manual expects the thermistor to read about 9,700 to 10,300 ohms at 25 degrees C, 2,400 to 2,600 ohms at 60 degrees C, and 1,000 to 1,100 ohms at 85 degrees C. Outside those ranges the sensor is replaced.
• Not a gas emergency: F5 is a sensor or wiring fault. If you ever smell gas or your CO alarm sounds, call the National Gas Emergency Service on 0800 111 999 straight away.
• Getting it fixed: a one-off repair visit runs Monday to Friday, 09:00 to 17:00, carries a £95 call-out fee paid in advance, and does not come with a workmanship guarantee. Smart Plan cover is a separate route, it is a service plan rather than insurance, and it is for future breakdowns rather than a fault your boiler is already showing. The full terms for both are set out at the end of this page.

Why has my Ideal boiler shown F5?

Because the boiler cannot tell the difference between a sensor that has failed and a sensor reporting something that makes no sense, F5 covers two situations. Either the thermistor or its wiring has gone, or the water is not moving through your system properly and the return reading is no longer plausible.

That second group is the reason F5 sometimes arrives with other symptoms, such as noisy running, cold radiators or the boiler firing briefly and stopping again. The symptom column below is what tells the causes apart, so it is worth noting what your boiler is actually doing before you ring anyone.

What could be causing my F5, and who can fix it?

Can I fix an Ideal F5 myself?

Honestly, no. F5 is a sensor or wiring fault inside the boiler, and every repair for it sits behind a panel only a Gas Safe registered engineer may open. What you can do is reset once, and gather the handful of facts that make the engineer's visit quicker. That is the entire list, and padding it out would not be doing you any favours.

1. Reset the boiler once, exactly as your Ideal user guide describes. That is Ideal's own advice for F5. Do not keep resetting: repeated resets trigger a separate lockout on top of the original fault. Ideal's public code list ties that reset-count lockout to L5, while Ideal's own manuals use L5 for the return thermistor fault, so treat the code you see as model-specific and check your boiler's guide.

2. Read the pressure gauge and write the figure down. Ideal's normal cold reading is around 1.0 to 1.5 bar. A very low or very high reading is not what causes F5, but it is useful information for the engineer.

3. Check the room thermostat: working batteries, set above the current room temperature, and calling for heat. Then you can tell the engineer the boiler was genuinely being asked to fire.

4. Look over the outside of the boiler and the pipework you can see for obvious water leaks or damage. Do not open or remove any casing panel to do it.

5. Check whether your other gas appliances, such as the hob or a gas fire, and your other electrical items still work, so a wider gas or power supply problem can be ruled out.

6. If F5 comes back after that single reset, stop and book a Gas Safe registered engineer, quoting fault code F5.

That is the stop point, and here is why it is there. Everything else on this fault means opening a sealed gas appliance: removing the thermistor from the return pipe, testing it against Ideal's published resistance values, checking the loom back to the primary PCB, testing for 230V at internal connections, replacing the sensor or the board, working on the pump, or flushing the heat exchanger. All of it is Gas Safe registered engineer work.

The boiler has already shut itself down rather than fire without a reliable reading, so there is nothing to gain by resetting it again and again while you wait. If the house is cold, use electric heating in the meantime, and never bring a portable gas or paraffin heater indoors as a substitute, because those carry a carbon monoxide risk of their own.

F5 is not a gas escape. But if you ever smell gas, or your carbon monoxide alarm sounds, treat that as a separate emergency whatever the display says. Call the National Gas Emergency Service on 0800 111 999, open windows, turn the gas off at the meter if you can reach it safely, do not touch electrical switches, and leave the house.

Is my code really F5, or is it F4, L5 or F.5?

The codes either side of F5 look almost identical on a small display, and one of them is genuinely ambiguous even in Ideal's own paperwork. Here is how they differ, in Ideal's wording where Ideal publishes it.

F4 is the flow thermistor: the same type of sensor, but on the pipe leaving the boiler rather than the one coming back. Ideal's manuals pair them as "F4 or L4 - Flow Thermistor Fault". The symptoms and the repair are near identical to F5, so check the digit carefully before you quote it to anyone.

F6 is an outside sensor fault, meaning the weather-compensation sensor on an outside wall, which is only fitted on some systems rather than inside the boiler. F3 is a fan fault, a completely different component that gets misread simply because it sits next to F5 in every published table. F0 is the no water flow thermistor, another sensor code in the same family on Ideal's public list. F1 is low pressure, and it is the one Ideal code a homeowner can often clear themselves with the filling loop.

L5 is the awkward one, and it is the code most often mixed up with F5. Ideal's public fault-code page says L5 means "The boiler has been reset too many times in 15 minutes". Ideal's own Logic Code Combi ESP1 installation and servicing manual heads its section "F5 or L5 - Return Thermistor Fault". Both are Ideal documents and they disagree with each other, so treat L5 as model-specific and check the guide for your exact boiler rather than assuming either meaning. Our separate Ideal L5 page goes through that conflict in detail.

F.5, written with a dot, is not an Ideal code at all. Ideal writes its codes without a decimal point: F5, L2, C2. Dotted codes are Vaillant's format, so if your display shows F.5 you are most likely looking at a Vaillant boiler and this page is the wrong one for you.

When should I call a Gas Safe registered engineer?

As soon as F5 comes back after one reset. There is nothing else to try at home, and waiting will not clear it, because the boiler will not fire again until the reading it needs is reliable. Quote the code F5 when you book, and mention anything you noticed: cold radiators, noise, the pressure figure you wrote down.

The visit usually follows a set order, and it helps to know it so you can follow what is happening. The engineer removes the return thermistor from the central heating return pipe and tests its resistance with a multimeter against Ideal's published values. If the sensor reads correctly, they check its plug and the wiring loom back to the primary PCB. If that is sound too, Ideal's own fault-finding flow points at the PCB. Where F5 turned up alongside poor circulation, they will also look at the pump, the valves and whether the heat exchanger needs flushing.

All of that means working inside a sealed gas appliance, so it is Gas Safe registered engineer work every time, with no exceptions. You can check any engineer's registration on the Gas Safe Register before they start. Ask them to confirm which part they are replacing, and what it will cost, before the work goes ahead.

If you smell gas at any point, stop and call the National Gas Emergency Service on 0800 111 999. That call takes priority over any fault code on the screen.

Ideal F5 fault code FAQs

F5 on your Ideal boiler? We can get a Gas Safe registered engineer out to you.

Book a one-off repair and a Gas Safe registered engineer can test the return thermistor and its wiring, then talk you through what your boiler needs. One-off repair visits run Monday to Friday, 09:00 to 17:00. A £95 call-out fee applies to a one-off repair and is paid in advance, and one-off work does not carry a workmanship guarantee. The engineer confirms the price with you before any work goes ahead. Tell us when you book if you have no heating or hot water at all, or if there are young children, elderly or unwell people in the house. A Smart Plan boiler module is the other route, and it is for future breakdowns rather than a fault your boiler is already showing, so an F5 on the display today needs the one-off repair. Smart Plan is a service plan, not insurance, and you build your own cover by picking only the modules you want. 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. The repairs described on this page, from a thermistor or its wiring to a primary PCB, pump work or a heat-exchanger flush, all draw on that annual limit, and you pay anything above it. 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. Cover-plan call-outs run Monday to Friday, 08:00 to 18:00, and we aim to get an engineer out to you within two working days. Outside those hours an engineer attends only for a genuine emergency breakdown. 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. Call 0333 772 6247, Monday to Friday, 08:00 to 18:00, or email hello@smart-plan.com if you would rather talk it through first.

Helpful links

• Book a repair or build your cover
• All Ideal fault codes
• Ideal L5 fault code (the code F5 gets confused with)
• Ideal F1 low-pressure fault code
• Ideal F2 fault code
• Ideal Heating help hub
• Boiler repair
• Boiler & central heating cover
• Ideal's official boiler fault-code list
• Check an engineer on the Gas Safe Register