Ariston 723 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 723 fault code mean?
Fault code 723 indicates that the boiler has detected an overheat condition in heating Zone 3. The system's temperature sensor has registered temperatures beyond safe operating limits in that zone, causing the boiler to lock out automatically as a protective measure. This code appears on Ariston Clas Net One, Clas One, E-Combi, and E-System models — it is the Zone 3 equivalent of code 707 (which performs the same function on E-Combi Evo and Clas HE Evo models). If you see 723 on your display, the boiler will not restart normally until the underlying cause is identified and resolved. Do not repeatedly reset without investigating, as continued overheating can damage internal components.
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
- Circulation pump failure or reduced output Common
The pump is responsible for pushing hot water around all heating zones at the correct flow rate. If it has seized, worn, or is running below its rated speed, water in Zone 3 can stagnate and overheat rapidly. This is one of the most frequent triggers for zone-specific overheat faults on these Ariston models.
- Sludge or limescale restricting Zone 3 flow Common
Years of use allow iron oxide sludge and limescale to accumulate inside radiators, pipework, and the heat exchanger. A partial blockage in Zone 3 reduces water circulation, creating hotspots that the NTC sensor picks up and triggers the 723 lockout. Limescale is particularly common in hard-water areas of the UK.
- Faulty NTC temperature sensor giving false readings Sometimes
The NTC thermistor monitors water temperature and reports back to the PCB. If the sensor has developed a fault or its resistance has drifted out of range, it may report an overheat condition even when Zone 3 temperatures are actually normal. This produces a 723 lockout without genuine overheating occurring.
- Zone 3 motorised valve not opening correctly Sometimes
Motorised zone valves direct hot water to specific zones on demand. If the Zone 3 valve is sticking, partially closed, or has a failed actuator head, flow is restricted and heat builds up in that circuit, leading to an overheat event.
- Incorrect system pressure Sometimes
System pressure that is too low reduces the volume of water available to carry heat away from the heat exchanger. This can cause localised overheating, particularly in a zone that is already demanding heat. The filling loop can be used to top up pressure to the recommended 1–1.5 bar when cold.
- Faulty or incorrectly configured Zone 3 thermostat Rare
A thermostat that has failed internally, or one that has been set to an extremely high temperature, can cause the zone to call for heat continuously without ever satisfying demand. Prolonged heat demand without adequate dissipation contributes to overheating.
How to fix it
- Switch the boiler off and allow it to cool DIY safe
Turn the boiler off at the programmer or room thermostat and leave it for at least 15–20 minutes. Never attempt to open any boiler casing or touch pipework while components are still hot. Allowing the system to cool is a necessary safety step before anything else.
- Check your system pressure DIY safe
Look at the pressure gauge on the front of the boiler. The reading when the system is cold should sit between 1.0 and 1.5 bar. If it reads below 1.0 bar, use the filling loop (usually a braided flexible hose beneath the boiler with one or two isolation valves) to top up slowly until the gauge reaches around 1.2 bar, then close the filling loop valves firmly. Do not overfill beyond 2.5 bar.
- Attempt a single reset DIY safe
Press and hold the reset button for approximately 5 seconds — refer to your boiler's quick-start guide if you are unsure of its location. If the boiler fires up and runs normally, monitor it closely over the next hour. If code 723 reappears within a short period, do not reset again. Repeated resets without addressing the root cause can damage the heat exchanger.
- Check Zone 3 radiators for signs of restriction DIY safe
Feel along the radiators in Zone 3 while the heating is running (before it locks out again). Cold spots, particularly at the bottom of a radiator, suggest sludge. A radiator that stays cold entirely may indicate a closed lockshield valve or a stuck motorised valve. Report these observations to the engineer — it helps them diagnose the fault faster.
- Have a Gas Safe engineer inspect the circulation pump Gas Safe engineer
An engineer will check whether the pump is running, verify its speed settings, and test output pressure. A pump that has seized or is running below specification will need to be replaced. Pump replacement involves draining down part of the system and working on sealed central heating components — this must only be carried out by a qualified engineer.
- Have the NTC temperature sensor tested and replaced if faulty Gas Safe engineer
The engineer will use a multimeter to check the sensor's resistance against the manufacturer's specification at a known water temperature. A sensor reading outside the expected range must be replaced. The part itself is inexpensive (around £10–£30), but correct installation and re-commissioning require engineering knowledge.
- Have the Zone 3 motorised valve inspected Gas Safe engineer
The engineer will check whether the valve opens and closes fully on demand. A sticking valve can often be freed by replacing the actuator head alone, which is a relatively low-cost fix. A fully seized valve body will require replacement and a partial system drain-down.
- Consider a power flush if sludge is suspected Gas Safe engineer
If the engineer finds evidence of heavy sludge — cold radiator bottoms, dark water when bleeding, or a sluggish pump — a power flush may be recommended. This involves connecting specialist equipment to the system to circulate cleaning chemicals and dislodge debris under pressure. It is particularly worthwhile before investing in replacement parts, as sludge can quickly damage new components.
- Contact a Gas Safe registered engineer if the fault persists or you cannot resolve it yourself Gas Safe engineer
If the 723 code returns after a single reset, or if you cannot identify an obvious cause such as low pressure, you should book a Gas Safe registered engineer. Persistent overheating in Zone 3 indicates a mechanical or electrical fault that requires professional diagnosis. You can find registered engineers at gassaferegister.co.uk.
Parts you may need
- NTC temperature sensor (thermistor) · from £20
- Motorised zone valve actuator head · from £35
- Circulation pump · from £85
- Motorised zone valve body · from £55
- PCB (printed circuit board) · from £220
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
Is fault code 723 the same as code 707 on an Ariston boiler?
They represent the same type of fault — a Zone 3 overheat — but they appear on different Ariston model families. Code 707 is used on the E-Combi Evo and Clas HE Evo ranges, while code 723 appears on the Clas Net One, Clas One, E-Combi, and E-System models. If you see 723, you have the right page. The diagnostic and repair process is essentially identical regardless of which code your display shows.
Can I keep resetting the boiler to clear code 723?
You should limit yourself to one reset attempt. If the boiler locks out again quickly with the same code, further resetting will not fix anything and risks damaging the heat exchanger through repeated thermal stress. A persistent 723 means there is an underlying problem in Zone 3 that needs a Gas Safe engineer to diagnose properly.
How much does it typically cost to fix an Ariston 723 fault in the UK?
Most people with this fault pay somewhere between £120 and £380 depending on the cause. A straightforward sensor replacement tends to sit at the lower end, while a circulation pump or motorised valve replacement will be mid-range. A power flush, if sludge is the culprit, typically adds £300–£500 on top of the call-out. If the PCB turns out to be the root cause, that component alone can cost £200–£450 plus labour — but PCB failure is not a common trigger for this specific fault code.
What can I do to prevent the 723 fault from coming back?
Annual servicing by a Gas Safe engineer is the single most effective preventative measure — a service catches sludge build-up, pump wear, and sensor drift before they cause a lockout. If you live in a hard-water area, a magnetic system filter fitted to the return pipework will capture iron oxide particles before they accumulate. Keeping your system pressure between 1.0 and 1.5 bar and having the heating system balanced professionally also reduces the risk of zone-specific overheating.