Ariston 713 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 713 fault code mean?
Fault code 713 on an Ariston boiler indicates that the control board has detected a problem with the Zone 3 return sensor — the NTC thermistor responsible for monitoring the temperature of water coming back from your third heating zone. The PCB triggers this code when it receives a reading that is implausible, or when it detects an open or short circuit in that sensor circuit. Related codes 711 and 712 point to the same type of fault on Zone 1 and Zone 2 return sensors respectively, so if your display shows one of those instead, the diagnosis and repair process is essentially identical — just on a different zone's sensor. Code 713 is most commonly seen on multi-zone Ariston models such as the Clas Net ONE, Clas ONE, E-Combi, and E-System ranges. Until the fault is resolved the boiler will typically lock out to protect the system, so Zone 3 heating will be unavailable.
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
- Faulty or degraded NTC thermistor Common
The Zone 3 return sensor itself is the most likely culprit. NTC thermistors can fail over time due to age, corrosion, or prolonged exposure to high temperatures, causing their resistance to drift outside the range the PCB expects. When this happens the board flags the reading as invalid and locks out on code 713.
- Loose or disconnected wiring at the sensor Common
The connector between the NTC sensor and the wiring harness can work loose over time, particularly in a system that cycles frequently. A poor connection produces an intermittent or open-circuit signal that the PCB interprets as a sensor fault, even if the sensor itself is still functional.
- Damaged wiring harness Sometimes
Chafed, pinched, or heat-damaged cables running from the Zone 3 return sensor back to the PCB can cause a short circuit or broken circuit. This mimics a sensor failure and will trigger code 713 even when the thermistor is perfectly healthy. A Gas Safe engineer will check continuity along the full harness to rule this out.
- PCB fault Rare
If the sensor and its wiring both test within specification, the fault may lie with the control board itself — either a failed input channel or a broader PCB communication issue. This is less common but should be considered once the sensor and harness have been eliminated.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button for approximately five seconds. A single reset is a reasonable first response to any lockout code. If the boiler fires up and stays running, monitor it over the next few hours. If fault code 713 returns straight away, do not keep resetting — repeated resets without fixing the root cause can mask developing problems and, in some cases, cause further damage.
- Switch the boiler off and note down the fault code DIY safe
If the fault returns after your one reset attempt, switch the boiler off at the programmer or room thermostat and note the exact code displayed (711, 712, or 713) along with which zone has lost heating. This information will help the engineer diagnose the problem more quickly when they arrive.
- Contact a Gas Safe registered engineer for diagnosis Gas Safe engineer
A qualified engineer will visually inspect the Zone 3 return sensor and its wiring for signs of corrosion, physical damage, or loose connectors. They will then use a resistance meter to check whether the NTC thermistor's output values fall within the manufacturer's specified range at a known water temperature. If the sensor is out of range it will be replaced. If the sensor tests correctly, the engineer will perform a continuity check along the full wiring harness to look for a break or short.
- Allow the engineer to replace the faulty component Gas Safe engineer
Depending on the diagnostic results, the engineer will replace the NTC return sensor, repair or replace the wiring harness, or — if both test satisfactorily — investigate whether the PCB is at fault. NTC sensor replacement on an Ariston is generally a straightforward job; PCB replacement is more involved and may prompt a conversation about whether a new boiler represents better value.
Parts you may need
- Zone return NTC thermistor sensor · from £35
- Sensor wiring harness / loom · from £45
- Ariston PCB (control 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–£280, depending on the underlying cause.
Frequently asked questions
Can I fix Ariston fault code 713 myself?
Not safely. While you can carry out one reset and note down the code, the actual diagnosis and repair requires a Gas Safe registered engineer. Testing an NTC thermistor involves measuring its resistance and comparing it against manufacturer data, and replacing components on a gas appliance is legally restricted to Gas Safe registered professionals in the UK.
What is the difference between Ariston fault codes 711, 712, and 713?
All three point to a faulty return sensor (NTC thermistor), but on different heating zones. Code 711 relates to the Zone 1 return sensor, 712 to Zone 2, and 713 to Zone 3. The repair process is identical for all three — the engineer simply works on the sensor for the affected zone. If you have a single-zone system you are unlikely to see these codes; they appear on multi-zone models such as the Clas Net ONE, Clas ONE, E-Combi, and E-System.
How much does it cost to fix fault code 713 on an Ariston boiler?
Most homeowners pay between £120 and £280 all-in when the fault is caused by the NTC sensor or its wiring — this covers the engineer's time and the relatively low-cost part. If the PCB turns out to be the root cause, the board itself can cost £200–£300 in parts alone, pushing the total repair bill higher. In that scenario it is worth getting a new boiler quote alongside the repair quote, as a replacement PCB on an older boiler can represent poor value for money.
Will fault code 713 affect my whole heating system or just one zone?
Because code 713 is specific to the Zone 3 return sensor, only the Zone 3 circuit should be affected. Other zones and your hot water supply may continue to work normally, depending on how your system is configured. That said, the boiler will lock out to protect itself, so it is important not to ignore the fault for long — leaving a lockout unresolved can affect overall system performance and may mask developing issues elsewhere.