Ariston 705 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 705 fault code mean?
On Ariston Sensys and multi-zone system boilers, the display can show what appears to be '705' but is actually the two-part code '7 05', indicating a fault with the flow temperature sensor on zone 5 of the heating system. The boiler has detected that the zone 5 NTC thermistor is either reading outside its expected range or has lost communication with the PCB entirely. As a precaution, the boiler will lock out to prevent any damage to that heating circuit. If your display shows '7 05' split across two segments or lines, this page applies directly to your situation. Note that standard three-digit Ariston fault codes typically fall in other documented series (such as the 101–116 or 301–307 ranges); a true standalone '705' is not listed in Ariston's published UK fault code library, so the two-part '7 05' interpretation is the most likely match.
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 on zone 5 Common
The NTC flow sensor itself has developed an open or short circuit — the most common reason this code appears. Over time, heat cycling and moisture can degrade the sensor element until it stops returning a valid resistance reading to the PCB.
- Loose or corroded wiring at the sensor or PCB Common
The plug-in connector between the zone 5 flow sensor and the main PCB can work loose, or the terminals can corrode, especially in humid plant-room environments. This breaks the signal path without the sensor itself being faulty.
- Water ingress affecting the sensor or connector Sometimes
A minor leak near the zone 5 pipework or sensor housing can allow water to track along the cable and contaminate the connector, causing intermittent or permanent signal loss.
- Faulty PCB Rare
If the sensor and its wiring test fine with a multimeter, the fault may lie with the PCB's analogue input channel for zone 5. PCB failures are less frequent but should be considered once other causes are ruled out.
How to fix it
- Check that your boiler display genuinely reads '7 05' (two separate figures) rather than a different code DIY safe
Look carefully at the display. On most Ariston Sensys and multi-zone models, multi-part codes are shown with a brief gap or on separate segments. Confirm you have the right code before proceeding, as the repair path differs significantly for other fault codes.
- Reset the boiler once DIY safe
Locate the reset button on the control panel — on most Ariston models it is marked with a flame or a circular arrow symbol. Press and hold it for approximately 5–10 seconds until the boiler attempts to restart. A single reset is worthwhile in case the fault was a one-off sensor glitch. If the code returns promptly, do not keep resetting.
- Inspect for any visible leaks near the zone 5 pipework DIY safe
Without removing any panels or touching any components, look around the area of zone 5 for signs of dripping water or damp patches. If you find a leak, note its location to tell your engineer — this may be the root cause of the sensor fault.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
An engineer will use a multimeter to check the zone 5 NTC sensor's resistance against the manufacturer's temperature-resistance curve. They will also inspect the wiring loom and connectors for damage or corrosion. If the sensor is confirmed faulty, they will replace it. If the sensor and wiring check out, the PCB input channel will be investigated next.
Parts you may need
- Ariston zone flow NTC thermistor sensor · from £35
- Sensor wiring harness / connector · from £20
- Ariston main PCB (model-specific) · from £210
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–£320, depending on the underlying cause.
Frequently asked questions
Why does my Ariston boiler show '705' when that code isn't in the manual?
The code you are seeing is most likely the two-part code '7 05', which is used on Ariston's Sensys and multi-zone system boilers to flag a zone 5 flow sensor fault. The display can make the space between the two figures hard to spot, causing it to read as a single three-digit number. Standard Ariston fault codes sit in documented ranges such as 101–116 or 301–307; a genuine standalone '705' does not appear in Ariston's published UK code library. If you are unsure, photograph the display and show it to an Ariston-accredited engineer.
Can I fix the '7 05' zone 5 sensor fault myself?
The homeowner checks you can safely carry out are limited to a single reset and a visual inspection for obvious leaks. Testing and replacing the NTC thermistor, checking the PCB connector, or any other internal work must be done by a Gas Safe registered engineer. This is not a gas-safety restriction specifically — it is because incorrectly handling electrical components on the boiler can cause further damage and will void any remaining warranty.
How much will it cost to repair an Ariston '7 05' fault code?
For most households the repair falls in the range of £120–£320. This typically covers an engineer call-out and diagnostic fee (usually £60–£100) plus the cost of a replacement NTC sensor and the labour to fit it. If the fault turns out to be PCB-related, costs rise considerably — a replacement Ariston PCB is around £210 for the part alone, with labour on top, putting a PCB repair closer to £400–£600. If your boiler is over 10–12 years old and the PCB is implicated, it is worth asking your engineer whether a new boiler represents better long-term value.
Will the '7 05' fault affect all zones in my house or just zone 5?
In most cases the boiler will lock out entirely rather than continue running the remaining zones, because it cannot safely verify the temperature state of zone 5. This means your whole heating system may be without heat until the fault is resolved. Some Ariston multi-zone controllers allow the affected zone to be temporarily isolated via the system controller, but you should check with your engineer before doing this, as it may mask a more serious underlying problem.