Ariston 203 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 203 fault code mean?
Fault code 203 on Ariston boilers indicates that the control board (PCB) has lost its signal from the hot water cylinder's NTC temperature sensor — in plain terms, the sensor has gone open circuit and the boiler can no longer read the cylinder temperature. This sits within Ariston's 200-series codes, which all relate to the domestic hot water (DHW) circuit and solar thermal system. The closely related code 204 points to the same sensor but in a different failure mode: a short circuit rather than a loss of contact. Code 203 is classified as a non-volatile fault, meaning it will not clear on its own — the boiler needs either a manual reset or a repair before it will resume normal operation. Because this fault depends on a separate hot water storage cylinder, it only applies to Ariston system boilers paired with a cylinder; it will not appear on combi boilers.
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 (open circuit) Common
The cylinder temperature sensor itself has burned out or broken internally, resulting in infinite resistance and no readable signal reaching the PCB. This is the most frequent root cause and typically requires straightforward sensor replacement.
- Loose or disconnected wiring Common
The cable connecting the cylinder sensor to the boiler's control board may have worked loose at a connector, particularly if the system has been moved or serviced recently. Even a partial disconnection is enough to trigger the 203 code.
- Corroded or damaged wiring Sometimes
In damp installations such as lofts, garages, or utility rooms prone to condensation, wiring insulation can degrade and connections can corrode over time. This interrupts the low-voltage signal from the sensor to the PCB and mimics an open-circuit fault.
- Age-related sensor degradation Sometimes
NTC thermistors gradually drift out of their calibrated resistance-temperature range as they age, especially in systems that have not been serviced annually. Eventually the sensor's output becomes unreadable, triggering the fault even if it has not completely failed.
- PCB fault Rare
In rare cases the control board itself loses the ability to read the sensor input correctly, even when the sensor and wiring are in good condition. This is unusual and would normally only be suspected after the sensor and wiring have already been confirmed as healthy.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button on your Ariston boiler for approximately five seconds until the display clears, then allow the boiler to complete its restart sequence. If the 203 code returns straight away or within a short time, there is an underlying problem that a reset will not resolve. Do not keep resetting — repeated resets without addressing the root cause can mask the fault and cause additional wear.
- Check that your hot water cylinder sensor wire is visibly connected DIY safe
If you can safely access the area around your hot water cylinder (not the boiler's internal components), visually check that no cables have obviously come unplugged from the sensor pocket on the cylinder. Do not open the boiler casing or touch any internal wiring — this check is limited to the external sensor lead and its plug only.
- Call a Gas Safe registered engineer to test the sensor and wiring Gas Safe engineer
An engineer will use a multimeter to measure the resistance of the NTC thermistor across its temperature range. A healthy sensor produces a predictable, decreasing resistance as temperature rises; an open-circuit sensor will read as infinite resistance. The engineer will also inspect the wiring harness for corrosion, chafing, and loose terminals between the cylinder sensor and the PCB.
- Replace the cylinder temperature sensor if confirmed faulty Gas Safe engineer
If testing confirms the sensor has failed, the engineer will fit a compatible replacement NTC thermistor into the cylinder sensor pocket, reconnect and secure the wiring, and clear the fault code. The boiler is then run through a full domestic hot water cycle to confirm the repair.
- PCB investigation if sensor and wiring are both sound Gas Safe engineer
In the unlikely event that the sensor resistance and all wiring check out correctly, the engineer will investigate the PCB for faults on the sensor input circuit. PCB repair or replacement is a more involved job and will be quoted separately.
Parts you may need
- Cylinder NTC temperature sensor (Ariston compatible) · from £25
- Sensor wiring harness / connector loom · from £20
- PCB (control board) — if required · 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–£220, depending on the underlying cause.
Frequently asked questions
Is Ariston fault code 203 dangerous?
No, code 203 is not a safety-critical fault in the same way as a gas leak or carbon monoxide warning. The boiler has simply lost contact with the cylinder temperature sensor and will typically stop heating the hot water cylinder until the fault is resolved. You can continue to use central heating in the meantime, but do not ignore the fault — leaving a sensor fault unresolved can affect how efficiently your system runs.
What is the difference between Ariston fault codes 203 and 204?
Both codes relate to the same component — the NTC thermistor in your hot water cylinder — but they describe opposite failure modes. Code 203 means the sensor circuit has gone open circuit (broken connection or dead sensor), while code 204 means the sensor circuit has short-circuited. In practice, both faults are usually fixed in the same way: inspect the wiring first, then replace the sensor if needed.
Can I fix Ariston fault code 203 myself?
The only action a homeowner should take is a single boiler reset. Beyond that, diagnosing and replacing the cylinder temperature sensor involves accessing wiring and components inside or connected to a gas appliance, which must be carried out by a Gas Safe registered engineer. Attempting this work yourself could invalidate your warranty and is not legally permitted under UK gas safety regulations.
My Ariston boiler is still under warranty — will this repair be free?
Ariston offers manufacturer warranties ranging from two to twelve years depending on the model and registration. If your boiler is within its warranty period and has been serviced annually by a qualified engineer (as most warranties require), a sensor fault like code 203 should be covered. Contact Ariston customer support before booking an independent engineer, as using an unauthorised repairer for warranty work could affect your cover. Keep in mind that a PCB replacement, if ever needed, can cost £250–£400 or more, making warranty cover especially valuable for older boilers approaching the end of the standard term.