Ariston 704 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 704 fault code mean?
Fault code 704 tells you that the boiler has detected a problem with the NTC thermistor (temperature sensor) fitted to the return pipe of Heating Zone 3. This sensor's job is to measure the temperature of water coming back into the boiler from Zone 3; when it gives an implausible reading or stops responding altogether, the boiler logs code 704 and may shut down that zone or lock out entirely. The fault is essentially a sensor or wiring issue rather than a gas or combustion problem. Note: if you own an older Ariston Clas Net One, Clas One, E-Combi (non-Evo), or E-System (non-Evo) model and see code 713 on your display, the diagnosis and repair process is identical — 713 is simply the equivalent code for those model families. Related zone sensor codes follow the same pattern: 711 covers the Zone 1 return sensor and 712 covers the Zone 2 return sensor, so all three are diagnosed and fixed in the same way.
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
The NTC sensor itself is the most likely culprit. Over time these small components drift out of specification or fail outright, causing the boiler's PCB to receive readings that are out of range. A Gas Safe engineer can confirm failure quickly by measuring the sensor's resistance with a multimeter and comparing it to the expected value at a known water temperature.
- Loose or disconnected wiring at the sensor Common
Vibration, thermal cycling, and age can cause the connector at the sensor to work loose. Even a partial disconnection is enough to produce an erratic or absent signal and trigger code 704. An engineer will check the connector and the condition of the terminals before condemning the sensor itself.
- Damaged wiring loom between the sensor and PCB Sometimes
If the cable running from the Zone 3 return sensor back to the PCB has been chafed, pinched, or has suffered insulation breakdown, the signal will be corrupted. This is less common than a loose connector but still regularly found on older or heavily installed systems.
- PCB fault or communication error Rare
In a minority of cases the sensor and wiring test fine, and the fault lies with the PCB's input circuitry. The PCB may misinterpret a valid sensor signal, or an onboard component may have failed. PCB replacement is a last resort after all other causes have been ruled out.
How to fix it
- Check your boiler's warranty status before doing anything else DIY safe
Ariston boilers often carry manufacturer warranties of between 2 and 12 years. If your boiler is still within warranty, contact Ariston or your installer first — a covered repair costs you nothing and using an unauthorised engineer could void the warranty.
- Reset the boiler once DIY safe
Press and hold the reset button for approximately 5 seconds as described in your boiler's user guide. A single reset is reasonable — it clears any transient error. If code 704 reappears within a short time, do not keep resetting; repeated resets will not fix a hardware fault and may mask the problem from an engineer.
- Visually inspect the visible wiring around the boiler casing (homeowner check only) DIY safe
Without opening the boiler casing or touching any internal components, check whether any wiring you can see externally looks obviously damaged, burnt, or disconnected. Do not remove the boiler cover or touch internal wiring — this must be left to a qualified engineer.
- Arrange for a Gas Safe registered engineer to carry out a full diagnosis Gas Safe engineer
An engineer will remove the boiler casing, locate the Zone 3 return sensor, and test its resistance with a multimeter. They will also inspect the wiring loom and connectors for damage or corrosion. This is not a task for a homeowner — the sensor sits inside the boiler and involves handling electrical components on a gas appliance.
- Engineer to replace the Zone 3 NTC return sensor if confirmed faulty Gas Safe engineer
If the sensor's resistance readings are outside the manufacturer's specification, the engineer will fit a replacement NTC thermistor. This is a straightforward repair on most Ariston models and usually resolves the fault completely. The boiler should be tested across its full operating range after replacement.
- Engineer to inspect the PCB if sensor and wiring replacement does not clear the fault Gas Safe engineer
Should code 704 persist after a new sensor and any wiring repairs, the engineer will assess the PCB for signs of damage or component failure. PCB replacement is significantly more expensive, so a reputable engineer will exhaust all other options first.
Parts you may need
- NTC return temperature sensor (Zone 3) · from £20
- Sensor wiring loom / harness · from £35
- Boiler PCB (control board) · from £280
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
Is Ariston fault code 704 the same as code 713?
Yes, for practical purposes they are the same fault. Code 704 appears on the E-Combi Evo, E-System Evo, Clas HE Evo Combi, and Clas HE Evo System ranges, while code 713 is the equivalent on the older Clas Net One, Clas One, E-Combi (non-Evo), and E-System (non-Evo) models. Both indicate a defective Zone 3 return sensor, and the diagnosis and repair steps are identical.
Can I fix Ariston fault code 704 myself?
The only safe DIY steps are a single boiler reset and a visual check of any obviously accessible external wiring. The actual sensor is located inside the boiler casing, and testing or replacing it involves working on the internal electrics of a gas appliance. This must be carried out by a Gas Safe registered engineer. Attempting it yourself is unsafe and could also invalidate your boiler warranty.
How much will it cost to repair fault code 704?
Most people pay between £120 and £300 all-in, covering the engineer's call-out fee, labour, and the replacement NTC sensor (typically £10–£30 for the part). If a wiring loom also needs replacing, costs sit towards the top of that range. In the less common scenario where a PCB replacement is required on top, the total bill can rise to £400–£550 or more. Prices in London and the South East tend to run around 15–25% higher than the UK average.
Will fault code 704 stop my heating working completely?
It depends on how your system is configured. If Zone 3 is a standalone heating zone, only that zone is likely to be affected. Some Ariston models will continue to run other zones and hot water normally while logging the fault, whereas others may lock out entirely as a precaution. Either way, the fault should be investigated promptly — running a boiler with a defective sensor long-term can lead to inefficient operation or further component damage.