Ariston 702 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 702 fault code mean?
Fault code 702 on Ariston boilers has two distinct meanings depending on your model. On multi-zone models such as the E-Combi Evo, E-System EVO, Clas HE evo Combi and Clas HE evo System, it points to a problem with the Zone 2 flow or return temperature sensor — the component that monitors water temperature in the second heating zone (for example, where underfloor heating runs alongside radiators). On standard combi models without zone control, the same code can indicate a fault with the domestic hot water (DHW) flow sensor or turbine, meaning the boiler fails to detect when a tap is opened and will not attempt to ignite the burner. In either case the boiler is protecting itself by locking out rather than operating with unreliable temperature or flow data. Note: on Clas Net One, Clas One, E-Combi and E-System ranges, the equivalent code is 712 — check your display carefully to confirm which code you are seeing.
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
- Failed NTC thermistor (Zone 2 sensor) Common
The NTC temperature sensor monitoring Zone 2 flow or return has developed an open or short circuit, causing the boiler's PCB to receive an implausible or absent signal. This is the most frequent cause on multi-zone models and typically triggers a full lockout as a safety precaution.
- Blocked or seized DHW flow turbine Common
On standard combi models, limescale deposits or debris can jam the small paddle-wheel turbine in the hot water circuit. When the turbine cannot spin freely it sends no flow signal to the PCB, so the boiler never recognises that a tap has been opened.
- Loose or disconnected sensor wiring Sometimes
Vibration or poor contact over time can cause the wiring harness between the sensor and the PCB to work loose. The electrical fault looks identical to a dead sensor on diagnostics, so an engineer will always inspect connections before condemning the component itself.
- Corroded sensor connector or damaged cable Sometimes
Moisture inside the boiler casing — sometimes from a minor internal leak — can corrode the sensor connector pins or chafe the cable insulation, creating an intermittent or permanent fault signal.
- PCB fault misreading a healthy sensor Rare
Rarely, the control board develops a fault on the sensor input channel and reports code 702 even though the sensor and wiring are in good condition. An engineer can rule this in or out by temporarily substituting a known-good sensor.
How to fix it
- Identify your boiler model and which interpretation applies DIY safe
Check the model plate (usually inside the boiler casing door or on the appliance data label). If your system has two separate heating zones — for example underfloor heating and radiators controlled independently — you almost certainly have a Zone 2 sensor fault. If it is a standard combi with a single zone, the DHW flow turbine interpretation is more likely.
- Reset the boiler once DIY safe
Press and hold the reset button for approximately five seconds and allow the boiler to complete its start-up sequence. A one-off reset is reasonable; if fault code 702 returns within a short time, do not keep resetting as this will not resolve the underlying problem and repeated lockouts can stress other components.
- Check your gas supply is live DIY safe
Confirm other gas appliances in your home (cooker, hob) are working normally. If they are not, contact your gas supplier — there may be a wider supply interruption rather than a boiler fault.
- Check system pressure is within the normal range DIY safe
Look at the pressure gauge on your boiler. Normal operating pressure is typically between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, top up via the filling loop following your boiler's user guide. Low pressure alone will not cause a 702 fault but is worth ruling out before calling an engineer.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
All internal work on the boiler — including testing sensor resistance with a multimeter, inspecting the PCB connections, cleaning or replacing the DHW flow turbine, and fitting a new NTC thermistor — must be carried out by a Gas Safe registered engineer. Do not attempt to open the boiler casing or interfere with gas or electrical components yourself. Most Ariston sensor repairs are completed in a single visit as engineers typically carry common temperature probes and flow turbine assemblies.
Parts you may need
- Ariston NTC temperature sensor / thermistor · from £20
- DHW flow turbine / flow sensor assembly · from £35
- Sensor wiring harness · from £18
- Zone 2 sensor connector / repair kit · from £10
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–£280, depending on the underlying cause.
Frequently asked questions
How do I know whether my Ariston 702 fault is the Zone 2 sensor or the DHW turbine?
The quickest way is to check your system layout. If your heating is split into two independently controlled zones (a common setup with underfloor heating downstairs and radiators upstairs), the fault is almost certainly the Zone 2 temperature sensor. If you have a standard single-zone combi with no zone controller, and the problem is that hot water stops working when you open a tap, the DHW flow turbine is the more likely culprit. Your Gas Safe engineer can confirm with a multimeter test in minutes.
Can I fix Ariston fault 702 myself?
The safe DIY steps are limited to a single reset attempt and confirming your gas supply and system pressure are normal. Any internal work — swapping an NTC sensor, cleaning or replacing the flow turbine, or testing wiring at the PCB — must be done by a Gas Safe registered engineer. Attempting this yourself is illegal on a gas appliance and could invalidate your warranty and home insurance.
What does fault code 712 mean on my Ariston boiler — is it the same as 702?
On Clas Net One, Clas One, E-Combi and E-System model ranges, code 712 is the equivalent of 702 on other Ariston families. It relates to the same type of sensor fault and is diagnosed and repaired in the same way. Double-check the digits on your display — 702 and 712 are easy to misread — and mention both to your engineer so they can cross-reference the correct component for your specific model.
My Ariston boiler is showing 702 and my hot water has stopped completely — will it be expensive to fix?
For the vast majority of homeowners this is a sensor or flow turbine replacement, which typically costs between £120 and £280 all-in (callout, labour, parts, and VAT). NTC thermistors are inexpensive components — often under £25 — and flow turbines are similarly affordable. The bill is mainly the engineer's time. If the fault turns out to be PCB-related, costs can rise to £400 or more, but this is uncommon for a 702 code. If your boiler is over ten years old and quotes are coming in at the higher end, it is worth getting a new boiler comparison price at the same time.