Ariston 701 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 701 fault code mean?
Fault code 701 on Ariston boilers signals a problem with a flow or outgoing temperature sensor, though the exact sensor involved depends on your specific model. On Clas Net One, Clas One, E-Combi, and E-System boilers it points to a defective Zone 1 flow (send) sensor — the thermistor that monitors the outgoing water temperature on your primary heating circuit. On E-Combi Evo, E-System Evo, Clas HE Evo Combi, and Clas HE Evo System models it instead flags a fault with the Zone 2 outgoing sensor. A small number of Ariston models use code 701 to indicate a DHW (domestic hot water) temperature sensor fault. In all cases, the boiler has detected that a sensor reading is missing, implausible, or outside the expected range, and it has locked out to protect the system. Confirm which definition applies to your boiler before proceeding — check the model name on the front panel and cross-reference with your user manual.
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 or degraded NTC thermistor Common
NTC thermistors are small, low-cost temperature sensors that age over time. The resistance they produce drifts outside acceptable limits, causing the boiler's PCB to log a sensor fault. This is by far the most common root cause of a 701 code — the sensor itself has simply reached the end of its service life.
- Loose or corroded sensor wiring Common
The wiring harness connecting the sensor to the PCB can work loose due to vibration, or the connector pins can corrode, especially in older or poorly ventilated boiler cupboards. A poor connection produces an intermittent or open-circuit signal that the PCB interprets as a sensor failure.
- Physical damage to the sensor Sometimes
The sensor probe or its cable may have been damaged — for example, by rubbing against pipework, overheating from nearby components, or accidental contact during a previous service. A damaged sensor will typically produce a fixed, incorrect reading or no reading at all.
- PCB fault misreading a healthy sensor Rare
If the sensor and its wiring test within normal parameters, the fault may lie with the PCB itself, which can fail to process the sensor signal correctly. This is less common but should be considered once the sensor and wiring have been ruled out.
How to fix it
- Check which zone sensor is flagged for your model DIY safe
Before doing anything else, confirm your boiler model (printed on the front casing) and look up whether code 701 on your specific model refers to Zone 1, Zone 2, or the DHW sensor. This tells your engineer exactly where to focus and avoids unnecessary diagnostic time.
- Attempt a single boiler reset DIY safe
Press and hold the reset button — usually on the main control panel or behind a small flap — for approximately 5 seconds until the boiler attempts to restart. If the 701 code clears and the boiler runs normally, monitor it over the next hour. If the code returns, do not keep resetting, as repeated resets without fixing the underlying fault can mask the problem and potentially cause further damage.
- Check your boiler's warranty status DIY safe
Ariston warranties range from 2 to 12 years depending on model and registration. If your boiler is still within its warranty period, contact Ariston's UK customer support before arranging an independent engineer — a covered repair could save you the full cost of the job.
- Have a Gas Safe engineer inspect the sensor and wiring Gas Safe engineer
A registered engineer will use a multimeter to test the NTC thermistor's resistance against the expected temperature-resistance curve. They will also check the wiring harness and connectors for continuity, corrosion, and secure fitment. This diagnostic work must be carried out by a Gas Safe registered professional — do not attempt to access internal boiler components yourself.
- Replace the faulty NTC thermistor if confirmed defective Gas Safe engineer
If testing confirms the sensor is out of specification, the engineer will fit a replacement Ariston-compatible NTC thermistor for the relevant zone. The part itself is inexpensive (typically under £30), so the majority of the repair cost is labour. The engineer should reset the fault code after fitting and verify the sensor reading on the PCB before leaving.
- Repair or replace damaged wiring if wiring is the cause Gas Safe engineer
If the sensor tests within spec but the connection or cable is damaged or corroded, the engineer will repair or replace the relevant section of the wiring harness. Connector corrosion can sometimes be cleaned and reseated, but badly degraded wiring should be replaced rather than repaired temporarily.
- Arrange PCB investigation if sensor and wiring are both healthy Gas Safe engineer
If neither the sensor nor the wiring is at fault, the PCB may be misreading the signal. PCB diagnosis and replacement is specialist work and significantly more expensive — your engineer should confirm this diagnosis before committing to a PCB replacement, and you may wish to get a second opinion given the cost involved.
Parts you may need
- Ariston NTC flow temperature thermistor (Zone 1 / Zone 2) · from £22
- Ariston DHW NTC temperature sensor · from £25
- Sensor wiring harness / connector loom · from £35
- Ariston PCB (if sensor diagnosis fails) · from £180
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–£300, depending on the underlying cause.
Frequently asked questions
Can I reset an Ariston 701 fault code myself?
Yes, a single reset attempt is perfectly reasonable. Hold the reset button for around 5 seconds and see whether the boiler runs normally afterwards. If the 701 code comes back within a short time, stop resetting and book an engineer — the underlying sensor or wiring fault needs proper diagnosis rather than repeated resets.
Is fault code 701 the same on all Ariston boilers?
No — and this is an important point. On Clas Net One, Clas One, E-Combi, and E-System models, code 701 refers to the Zone 1 flow sensor. On E-Combi Evo, E-System Evo, and Clas HE Evo models it points to the Zone 2 outgoing sensor. A small number of models use 701 for the DHW temperature sensor. Always check your model name before speaking to an engineer so they know which sensor to test.
How much does it typically cost to fix an Ariston 701 fault?
For the most common repair — replacing a faulty NTC thermistor and call-out — most homeowners pay between £120 and £300 including parts, labour, and VAT. The sensor component itself is usually under £30, so the bulk of the cost is the engineer's call-out and time. If the PCB turns out to be the culprit, costs can rise significantly — a replacement PCB can add £200–£400 to the bill — but this is an uncommon outcome and should only be considered once sensor and wiring have been properly ruled out. Prices in London and the South East typically run around 15–25% higher than the UK national average.
Will my boiler keep working with a 701 fault code showing?
In most cases, no. Ariston boilers will lock out when a sensor fault is detected, meaning central heating and/or hot water will stop working until the fault is resolved and the code is cleared. Do not attempt to force repeated restarts — arrange a Gas Safe engineer to diagnose and repair the fault.