Ariston 703 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 703 fault code mean?
Fault code 703 appears on certain Ariston boilers to indicate that the temperature sensor monitoring the outgoing (flow) water on heating Zone 3 has either failed or is returning a signal outside the expected range. This sensor — an NTC thermistor — works by changing its electrical resistance in proportion to temperature: as the water heats up, the resistance drops, and the boiler's PCB uses this reading to regulate how much heat is produced. When the PCB detects that this sensor's signal is absent, erratic, or implausible, it raises the 703 fault and suspends normal operation to prevent uncontrolled heating on Zone 3. The fault points specifically at Zone 3's flow sensor rather than a general sensor failure, so it is most relevant to multi-zone Ariston system installations.
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 sensor itself is the most frequent culprit. NTC thermistors can drift out of calibration over time, develop internal breaks, or simply fail with age. When the resistance value it sends to the PCB falls outside acceptable limits, the boiler logs 703 and locks out.
- Loose or disconnected wiring at the sensor connector Common
Vibration, thermal cycling, or a previous service job can leave sensor connector plugs partially seated or unplugged entirely. Even a slightly loose connection can produce an intermittent or out-of-range resistance reading that triggers the fault.
- Corroded or damaged connector terminals Sometimes
Moisture ingress inside the boiler casing — sometimes from a minor internal leak — can corrode the small pin terminals at the sensor connector. Corrosion adds resistance to the circuit and can mimic a faulty sensor without the sensor itself being defective.
- Short circuit or open circuit in the sensor wiring harness Sometimes
A break or short in the wiring between the sensor and the PCB disrupts the resistance signal entirely. This can result from chafing against sheet metal edges, rodent damage in older installations, or a previous repair carried out carelessly.
- PCB fault Rare
If the sensor and its wiring both test within specification, the PCB's own sensor input circuit may be at fault. The PCB interprets the incoming resistance value; a damaged input channel can misread a perfectly healthy sensor as defective.
How to fix it
- Try a single boiler reset DIY safe
Locate the reset button on your Ariston boiler's control panel — on most models it is a dedicated button or a long-press function on the mode dial. Hold it for approximately five seconds until the boiler attempts to restart. If the 703 code returns immediately or within a short time, do not keep resetting; repeated resets on a genuine sensor fault will not fix the underlying problem and can mask it from an engineer.
- Check the boiler's warranty status before spending anything DIY safe
Ariston typically offers manufacturer warranties ranging from two to twelve years depending on the model and registration. Log into the Ariston UK website or check your installation documents to see whether your boiler is still covered. If it is, contact Ariston or your installer before authorising any paid repair, as covered faults should be rectified at no cost.
- Call a Gas Safe registered engineer to diagnose the fault Gas Safe engineer
A qualified engineer will open the casing, locate the Zone 3 flow sensor, and use a multimeter to measure its resistance at a known water temperature. A healthy NTC thermistor follows a predictable resistance-temperature curve; a reading significantly outside that curve confirms sensor failure. The engineer will also inspect the wiring harness and connector terminals for looseness, corrosion, or damage before condemning the sensor itself.
- Engineer to repair or replace the faulty component Gas Safe engineer
If the sensor is confirmed defective, the engineer will fit a replacement NTC thermistor for Zone 3 — a relatively straightforward parts-and-labour job. If wiring or terminals are corroded or broken, these will be repaired or replaced. Only if both the sensor and wiring test sound will the engineer investigate the PCB's sensor input circuitry as the root cause.
Parts you may need
- NTC flow temperature sensor (Zone 3) · from £25
- Sensor connector wiring loom / harness section · from £35
- Boiler PCB (if sensor input circuit is at fault) · 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 £100–£300, depending on the underlying cause.
Frequently asked questions
Can I keep using my Ariston boiler while it is showing fault code 703?
In most cases the boiler will lock out and stop heating when this code is active, so continued use is not possible until the fault is cleared. Even on models that limp along in a reduced mode, running the boiler without a functioning Zone 3 flow sensor means the PCB cannot properly regulate water temperature on that zone, which risks overheating. It is best to get it looked at promptly rather than attempting workarounds.
Is fault code 703 expensive to fix?
For the majority of homeowners the repair involves either a wiring fix or a new NTC sensor, and total parts-plus-labour costs typically fall between £100 and £300. If the PCB's sensor input channel turns out to be the underlying problem, replacement PCBs can cost £400–£600 all-in — at that point it is worth getting a quote for a new boiler, especially on an older unit, as modern replacements are significantly more energy-efficient.
Why has my boiler developed a 703 fault after a service?
Occasionally a sensor connector is inadvertently disturbed during a service — the plug may not have been fully re-seated, or a wiring clip dislodged. If the 703 fault appeared shortly after servicing work, contact the engineer who carried it out; reseating a loose connector is usually a quick revisit job and reputable engineers will return at no extra charge to rectify something caused during their own work.
Does the 703 fault mean my entire boiler needs replacing?
Not usually. A defective Zone 3 flow sensor is a relatively minor component failure and the vast majority of cases are resolved with a sensor swap or a wiring repair. Replacement only becomes a serious consideration if the PCB is also found to be faulty on a boiler that is already old and out of warranty, where the combined repair cost approaches or exceeds the cost of a new energy-efficient model.