Ariston 204 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 204 fault code mean?
Fault code 204 on Ariston boilers signals that the NTC thermistor (temperature sensor) monitoring the hot water storage cylinder has developed a short circuit. Rather than sending a steady, reliable temperature reading, the sensor is producing a signal that the boiler's control board identifies as a short — so the boiler locks out to protect the system. On Ariston models fitted with solar thermal, code 204 can also point to a short-circuit fault in the solar collector temperature probe. Code 204 sits within the 200-series range, which covers domestic hot water circuit and solar system sensor faults. For context, the closely related code 203 indicates the same cylinder sensor has gone open-circuit (no contact at all), whereas 204 specifically means a short circuit is detected.
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 (internal short circuit) Common
The sensor element itself has broken down internally, causing its resistance to collapse and produce a short-circuit signal. This is the most direct cause of code 204 and almost always requires the sensor to be replaced.
- Damaged or pinched sensor wiring Common
The cables running between the cylinder sensor and the boiler can become chafed, pinched, or corroded over time. If two conductors make unintended contact, the control board reads this as a short circuit even when the sensor itself is still intact.
- Loose or poorly seated wiring connections Sometimes
Connector plugs at either the sensor end or the PCB end can work loose, corrode, or sit slightly proud of their terminals. An intermittent or degraded connection can trigger the same short-circuit error code.
- Corrosion or water ingress at the sensor Sometimes
In older systems, moisture can track along cable runs and reach the sensor body or its connector, corroding the contacts and causing erratic or short-circuit readings over time.
- Solar collector probe fault (solar-equipped models only) Rare
On Ariston system boilers with an integrated solar thermal input, code 204 may instead indicate a short circuit in the solar collector temperature probe rather than the cylinder sensor. The diagnostic approach is the same but the probe location differs.
How to fix it
- Attempt a single boiler reset DIY safe
Press and hold the reset button for approximately five seconds until the boiler attempts to restart. If code 204 clears and does not return within a short time, monitor the boiler over the next hour. If the fault reappears — which is likely with a genuine sensor failure — do not keep resetting, as repeated resets will not resolve an underlying electrical fault.
- Check that power and hot-water controls are set correctly DIY safe
Confirm the boiler is powered on, the programmer or timer is set to call for hot water, and no other fault codes are displayed alongside 204. Rule out any simple controls issue before concluding the fault is hardware-related.
- Engage a Gas Safe registered engineer to inspect the sensor wiring Gas Safe engineer
An engineer will trace the wiring loom from the cylinder NTC sensor back to the boiler, checking for chafed insulation, pinched cables, corroded connectors, and loose terminal plugs. Many code 204 faults are resolved at this stage simply by reseating or replacing a connector.
- Engineer to test the NTC sensor resistance Gas Safe engineer
Using a multimeter, the engineer will measure the sensor's resistance at a known temperature and compare it against Ariston's published reference values. A reading near zero ohms confirms an internal short circuit and means the sensor must be replaced.
- Engineer to replace the faulty NTC thermistor or solar probe Gas Safe engineer
The engineer will fit a compatible replacement NTC sensor to the cylinder (or solar collector, if applicable), reconnect the wiring correctly, and verify the new sensor is producing plausible temperature readings before clearing the fault code and running a functional test.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
Code 204 cannot be resolved without hands-on electrical diagnosis and component replacement. If the reset in step one did not clear the fault, contact a Gas Safe registered engineer to carry out the inspection and repair described above. Always confirm your engineer is Gas Safe registered before they begin work.
Parts you may need
- Cylinder NTC temperature sensor (thermistor) · from £25
- Sensor wiring harness / loom · from £35
- Sensor connector plug and terminals · from £12
- Solar collector temperature probe (solar models only) · from £40
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 £150–£300, depending on the underlying cause.
Frequently asked questions
Can I fix Ariston fault code 204 myself?
No. While you can safely attempt a single reset, the actual repair — inspecting wiring, testing sensor resistance, and replacing the NTC thermistor — must be carried out by a Gas Safe registered engineer. Working on boiler electrical components without the correct registration is both unsafe and illegal in the UK.
What is the difference between Ariston fault codes 203 and 204?
Both codes relate to the cylinder temperature sensor (NTC thermistor), but they indicate opposite electrical faults. Code 203 means the sensor circuit is open — there is no electrical contact, as though the wire has been cut. Code 204 means the sensor circuit has short-circuited — the resistance has collapsed to near zero. Both require a Gas Safe engineer to diagnose and repair, typically by checking wiring connections and replacing the sensor if it has failed.
How much does it cost to fix Ariston fault code 204 in the UK?
Most homeowners pay between £150 and £300 for this repair. The cylinder NTC sensor itself is one of the less expensive boiler parts — typically £15 to £50 — so the bulk of the bill is the engineer's call-out and labour time, which usually runs to one to two hours on a weekday. Emergency or weekend call-outs cost more. In the rare event that wiring damage is extensive or a PCB has been affected by a persistent short circuit, costs could be higher, but this is uncommon for a straightforward sensor fault.
My Ariston boiler has a solar thermal system — could code 204 be a solar fault rather than a cylinder fault?
Yes. On Ariston models with an integrated solar thermal input, fault code 204 can indicate a short circuit in the solar collector temperature probe rather than the domestic hot water cylinder sensor. The two components serve different parts of the system but trigger the same code. A Gas Safe engineer familiar with solar-assisted boilers will be able to identify which probe is at fault during their inspection.