Ariston 205 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 205 fault code mean?
Fault code 205 on Ariston boilers signals that the domestic hot water (DHW) inlet temperature probe has lost its electrical connection to the boiler's control board — in other words, the circuit between the sensor and the PCB is broken (open circuit). On models linked to a solar thermal system, this is specifically the temperature sensor positioned at the solar-side inlet of the DHW circuit. Without a reliable signal from this probe, the boiler cannot accurately monitor incoming hot water temperatures and locks out to protect the system. Ariston groups all DHW (sanitary hot water) circuit faults under the 2xx series: code 205 means the probe has gone open circuit, while the closely related code 206 means the same probe has short-circuited. Both point to the same sensor and wiring loom, so the diagnostic approach is identical.
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
- Loose or disconnected probe wiring Common
Vibration over time can work a connector loose from the DHW inlet probe or from its socket on the PCB wiring loom. This is the most straightforward cause and is often resolved simply by reseating the connector — though this must be done by an engineer with the boiler isolated.
- Failed NTC temperature probe Common
The NTC (negative temperature coefficient) thermistor that monitors DHW inlet temperature can degrade or fail completely, particularly in older boilers or those that have not been serviced regularly. A failed sensor produces no signal, which the control board interprets as an open circuit.
- Damaged or corroded wiring loom Sometimes
Moisture ingress, heat damage, or simply age can cause the insulation on the wiring between the probe and the PCB to crack or corrode, breaking electrical continuity. This is more common on boilers installed in damp or poorly ventilated locations.
- PCB connection or control board fault Rare
If the wiring and probe both test correctly, the fault may lie at the PCB itself — a corroded terminal, a dry solder joint, or a damaged input channel on the control board can mimic an open-circuit sensor reading.
How to fix it
- Check your boiler is within its warranty period before arranging a repair DIY safe
Ariston boilers typically come with a manufacturer's warranty of between 2 and 12 years depending on the model and registration. If your boiler is still covered, contact Ariston or your installer before paying for any repair, as the work may be carried out at no cost to you.
- Attempt a single reset DIY safe
Press and hold the reset button for approximately 5 seconds. If the boiler fires up and runs normally, monitor it closely over the next hour. If fault code 205 returns, do not keep resetting — repeated resets without fixing the underlying fault will not help and may mask important diagnostic information for the engineer.
- Have a Gas Safe engineer inspect the DHW inlet probe and its wiring Gas Safe engineer
An engineer will isolate the boiler, locate the DHW inlet temperature sensor, and carry out a continuity and resistance check on the probe using a multimeter. NTC sensors have a known resistance curve against temperature, so a failed sensor is straightforward to identify. They will also check the wiring loom for damage, corrosion, or loose connectors along its full length.
- Have the engineer replace the NTC probe if it has failed Gas Safe engineer
If the sensor tests outside its expected resistance range or shows no continuity, replacement is straightforward in most cases. Ariston NTC probes are widely available as genuine or quality aftermarket parts. The engineer will fit and test the new probe before returning the boiler to service.
- Have the engineer inspect the PCB if the probe and wiring are both sound Gas Safe engineer
Should the probe and loom check out correctly, the engineer will examine the PCB terminal connections for the DHW sensor input. Corroded pins or a damaged board input channel can produce a false open-circuit reading. Depending on findings, the board may need cleaning, repairing, or replacing.
Parts you may need
- Ariston DHW inlet NTC temperature probe · from £18
- Wiring loom / sensor harness · from £35
- Ariston PCB (control board) · 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–£280, depending on the underlying cause.
Frequently asked questions
What is the difference between Ariston fault codes 205 and 206?
Both codes relate to the same DHW inlet temperature probe, but they describe opposite electrical faults. Code 205 means the probe circuit is open — there is a break in the connection somewhere between the sensor and the PCB. Code 206 means the circuit has short-circuited — the probe or its wiring has developed a direct short. The diagnostic approach is the same for both: an engineer tests the sensor's resistance and checks the wiring loom. The repair is usually identical too, typically a wiring fix or a probe replacement.
Can I fix Ariston fault code 205 myself?
The only homeowner-safe steps are checking your warranty, attempting one reset, and noting whether the fault returns immediately. The probe, its wiring, and the PCB are all internal boiler components that must only be accessed by a Gas Safe registered engineer. Attempting to work on them yourself is unsafe and may also invalidate your boiler warranty.
How much does it cost to fix fault code 205 on an Ariston boiler?
For the most common repairs — reseating a connector or replacing the NTC probe — you should expect to pay roughly £100–£280 including parts and labour. A straightforward probe swap is towards the lower end of that range. If the PCB is found to be at fault, a replacement board for an Ariston boiler typically costs £150–£250 in parts alone, pushing the total repair higher; however, PCB faults are an uncommon cause of this specific code.
Will my Ariston boiler still provide heating when fault code 205 is displayed?
That depends on how the boiler locks out. In many cases a DHW probe fault will prevent the boiler from operating in hot water mode but may allow central heating to continue. However, some Ariston models will go to a full lockout, disabling both heating and hot water. Check your display for any heating output and call an engineer promptly either way, especially in cold weather.