Chaffoteaux 723 Fault Code: Causes, Fixes & Repair Costs
What does the Chaffoteaux 723 fault code mean?
Fault code 723 on a Chaffoteaux boiler indicates that an overtemperature condition has been detected in Zone 3 of the heating system. Chaffoteaux uses a structured numbering system for its fault codes: the leading digit (7) identifies the multizone hydraulic circuit group, while the trailing digits (23) pinpoint Zone 3 specifically. In plain terms, the boiler's control electronics have registered that water temperature in Zone 3 has exceeded safe limits, and the boiler has locked out as a safety precaution. This code appears on multizone-capable Chaffoteaux models such as those in the Mira Advance and Pigma Green ranges. If left unaddressed, repeated overtemperature events can cause premature wear to components, so it is important to identify the root cause rather than simply resetting the boiler repeatedly.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty Zone 3 temperature sensor (NTC thermistor) Common
The NTC thermistor responsible for monitoring Zone 3 water temperature can drift out of specification or fail entirely, causing it to send inaccurate readings to the control board. The board may interpret these incorrect readings as an overtemperature event even when the water is within a safe range. A broken connection between the sensor and the PCB connector can produce the same symptom. In some cases the boiler will self-recover after a few seconds if the fault was a momentary contact issue, but persistent lock-outs strongly suggest the sensor needs testing and likely replacement.
- Restricted circulation in Zone 3 Common
If water cannot flow freely around Zone 3, heat builds up and the temperature sensor correctly detects a genuine overtemperature event. Common causes of restricted flow include a partially seized or failing zone circulator pump, a stuck or faulty zone valve, and air locks in the pipework. Cold or unevenly heated radiators on Zone 3, along with noise from the pipework or pump, are tell-tale signs that circulation is compromised.
- Sludge or debris build-up in the Zone 3 circuit Sometimes
Years of use can allow magnetite sludge and limescale deposits to accumulate inside the pipework, radiators, and heat exchanger passages serving Zone 3. This restricts flow in much the same way as a mechanical blockage, causing localised overheating. If radiators on Zone 3 are cold at the bottom but warm at the top, or if the boiler has not had a system inhibitor added for several years, sludge is a likely contributor.
- Moisture or poor connections at the PCB or sensor connector Sometimes
Condensation or water ingress on the Zone 3 sensor connector or at the corresponding terminal on the control board can create false resistance readings, mimicking a failed or overheating sensor. Inspecting the connectors for corrosion, moisture, or pushed-back pins is a standard first step a Gas Safe engineer will take before condemning the sensor itself.
- Incorrect zone wiring or configuration Rare
On multizone Chaffoteaux installations, wiring errors introduced during installation or after subsequent alterations can cause the control electronics to misinterpret signals from Zone 3 flow sensors. Although less common than sensor or circulation faults, this is worth investigating if the boiler is newly installed, has recently been altered, or if faults across multiple zones appear simultaneously.
How to fix it
- Check your gas supply is working DIY safe
Before doing anything else, confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier — there may be a supply interruption in your area. Do not attempt to open any gas valves or pipework yourself.
- Check system pressure and top up if needed DIY safe
Look at the pressure gauge on your boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1 bar, use the filling loop to repressurise the system to around 1.2 bar, then close the filling loop valves securely. Low pressure alone is unlikely to cause code 723, but it can compound circulation problems.
- Attempt a single boiler reset DIY safe
Locate the reset button on your Chaffoteaux boiler (refer to your manual if unsure of its position) and press it once. Wait for the boiler to complete its start-up sequence. If the fault clears and the boiler runs normally, monitor it over the next few hours. If fault code 723 returns, do not keep resetting — repeated resets without addressing the cause can mask a genuine safety issue and may use up the limited number of resets the boiler allows before requiring an engineer intervention.
- Note any symptoms to report to your engineer DIY safe
Before calling an engineer, take note of whether any radiators on Zone 3 are cold, whether you can hear unusual noises (banging, gurgling) from the pipework or pump, and how frequently the fault has appeared. This information helps the engineer diagnose the fault more quickly, potentially reducing the time — and cost — of the visit.
- Engineer: test the Zone 3 NTC thermistor resistance Gas Safe engineer
A Gas Safe engineer will use a multimeter to measure the resistance of the Zone 3 temperature sensor across the expected temperature range. If the readings fall outside the manufacturer's specification for that thermistor, the sensor is condemned and replaced. For Chaffoteaux models the relevant part (such as Ariston/Chaffoteaux thermistor 61000733) is widely available from UK spares suppliers, and the part cost itself is modest — typically under £15 — so the bulk of this repair is labour.
- Engineer: inspect Zone 3 pump and zone valve operation Gas Safe engineer
The engineer will check that the Zone 3 circulator pump is running freely and delivering adequate flow, and that any associated zone valve is opening and closing correctly. A pump that is seized or running but not delivering sufficient head will cause genuine overheating of Zone 3 water. Depending on findings, the pump or valve may be freed, serviced, or replaced.
- Engineer: check for sludge and advise on power flush if required Gas Safe engineer
If circulation is found to be impaired by magnetite sludge or scale deposits, the engineer may recommend a power flush of the Zone 3 circuit or the whole system. A power flush will also usually include the addition of a corrosion inhibitor to prevent recurrence. This is a more involved job and will be priced separately.
- Engineer: inspect PCB connectors and zone wiring Gas Safe engineer
If the sensor and pump check out as serviceable, the engineer will inspect the Zone 3 sensor connector and corresponding PCB terminal for moisture, corrosion, or poor contact. Wiring continuity across the zone circuit will also be verified, particularly on installations that have had zones added or altered. PCB replacement is a last resort and will be recommended only if other causes have been ruled out.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
If you have carried out the homeowner checks above and the boiler continues to display fault code 723, contact a Gas Safe registered engineer. You can verify any engineer's registration at GasSafeRegister.co.uk. All internal boiler work — including sensor replacement, pump work, and PCB inspection — is a legal requirement for a Gas Safe registered professional in the UK.
Parts you may need
- Zone 3 NTC thermistor (e.g. Ariston/Chaffoteaux 61000733) · from £13
- Zone circulator pump (compatible replacement) · from £85
- Zone valve actuator head · from £35
- System inhibitor (e.g. Fernox F1 or Sentinel X100, 1 litre) · from £18
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Can I keep resetting my Chaffoteaux boiler to clear fault code 723?
You can try a single reset as a first step, but you should not keep resetting the boiler if the fault returns. Firstly, Chaffoteaux multizone boilers limit the number of resets permitted before requiring a manual engineer intervention, so indiscriminate resetting may leave you without heating sooner. More importantly, code 723 signals a genuine overtemperature event or a sensor fault — neither of which is resolved by a reset alone. Persistent lock-outs need a Gas Safe engineer to identify and fix the underlying cause.
Could code 723 mean my Zone 3 thermistor needs replacing, and how much will that cost?
Yes, a failing NTC thermistor is the most common single cause of code 723. The part itself is inexpensive — widely available for around £10–£15 plus VAT — but you also need to pay for a Gas Safe engineer's time to test, diagnose, and replace it. Including the callout and labour, most homeowners pay in the region of £100–£200 for a straightforward thermistor replacement. If the engineer also finds a circulation or sludge problem that needs attention, costs will be higher.
My Zone 3 radiators are cold and I have fault code 723 — are these connected?
Very likely, yes. Cold radiators on Zone 3 alongside code 723 strongly suggest a circulation problem: the water in the zone cannot flow freely, so heat builds up near the boiler and triggers the overtemperature lock-out. The cause could be a failing or seized zone pump, a stuck zone valve, or sludge restricting flow in the pipework. A Gas Safe engineer will test the pump and valve operation and may recommend a power flush if sludge is present. A power flush typically costs £350–£500 in the UK but can significantly extend the life of the system.
My Chaffoteaux boiler is over ten years old and keeps showing fault 723 — is it worth repairing?
Chaffoteaux boilers generally have a service life of around 10–15 years with regular maintenance. If your boiler is within that range and the repair is limited to a thermistor or pump (typically £120–£380), repair is usually worthwhile. However, if the fault is traced to the PCB — which can cost £300–£500 or more to replace — and the boiler is already ageing, it is sensible to weigh repair cost against the price of a new, more efficient boiler. Your Gas Safe engineer can advise once they have diagnosed the root cause.