Ariston 112 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 112 fault code mean?
Fault code 112 appears on Ariston boilers when the control board detects a problem with the central heating return temperature probe circuit. This means the sensor fitted to the return pipe — the pipe that carries cooled water back into the boiler — is either reading an open circuit (broken connection) or a short circuit (unintended electrical contact). Because the boiler relies on this sensor to monitor water temperature accurately and operate safely, it locks out immediately when the signal is lost or unreliable. The probe itself is a small thermistor: a component whose electrical resistance changes in proportion to temperature, constantly feeding data to the PCB. When that data stream becomes erratic or disappears entirely, the boiler shuts down rather than risk overheating or damage.
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 return temperature probe (thermistor failure) Common
The thermistor itself degrades over time, particularly in older boilers or those that have seen heavy use. When the component's internal resistance drifts outside the expected range — either because it has broken down entirely or developed an internal short — the PCB can no longer interpret a valid temperature signal and triggers the 112 lockout. This is the most frequent root cause of this code.
- Loose or corroded wiring connections at the probe Common
The wires connecting the return probe to the PCB can work loose over time due to vibration, or the terminals can corrode — especially in boilers in damp plant rooms or utility spaces. A poor connection intermittently breaks the circuit, which the PCB interprets as a fault. This sometimes explains why 112 appears and then clears on its own for a while before returning permanently.
- Damaged wiring loom between probe and PCB Sometimes
If recent plumbing or heating work has been carried out nearby, the wiring to the probe may have been snagged, kinked, or chafed against a pipe or bracket. Physical damage to the wire insulation can cause short circuits, while a broken conductor creates an open circuit — both triggering code 112.
- PCB fault causing incorrect sensor readings Rare
In a small number of cases the probe and its wiring test as perfectly healthy, but the PCB input channel that processes the sensor signal has failed. This is less common but should be considered if a new probe and fully inspected wiring still result in the same fault code returning.
How to fix it
- Check your gas supply is working DIY safe
Before assuming the boiler is at fault, confirm that other gas appliances in your home — a hob or a gas fire — are working normally. If there is no gas reaching the property, contact your gas supplier rather than an engineer.
- Reset the boiler once DIY safe
Press and hold the reset button (refer to your boiler's user manual for its exact location) until the boiler attempts to restart. Allow it a full heating cycle to see whether the fault clears. If 112 reappears, do not keep resetting — repeated resets will not fix an underlying sensor or wiring fault and may mask a developing problem.
- Visually check the visible wiring without removing the boiler casing DIY safe
Look at any external pipework and wiring visible around the boiler without opening any panels. If you can see an obviously damaged or disconnected cable, note it and mention it to the engineer. Do not attempt to open the boiler casing, touch internal components, or probe any wiring yourself.
- Contact a Gas Safe registered engineer to test the return probe circuit Gas Safe engineer
An engineer will remove the boiler casing safely and use a multimeter to measure the resistance of the return temperature probe and check the continuity of its wiring loom. This confirms whether the thermistor itself has failed or whether the fault lies in the wiring or connections. While on site, it is worth asking the engineer to check the flow probe at the same time, as both sensors are similar in age and condition.
- Replace the return temperature probe if confirmed faulty Gas Safe engineer
If the thermistor is outside its specified resistance range, the engineer will fit a replacement probe. This is a relatively straightforward job on most Ariston models — the sensor clips or screws onto the return pipework and the connector plugs into the loom. The boiler can usually be returned to service the same visit.
- Inspect and repair wiring or connectors if the probe tests healthy Gas Safe engineer
When the probe itself is functioning correctly, the engineer will trace the wiring back towards the PCB, checking for corrosion at terminals, chafing, or breaks in the conductor. Connectors can be cleaned or replaced and damaged sections of loom repaired or bypassed.
- Consider PCB investigation if the fault persists after probe and wiring checks Gas Safe engineer
If a new probe and fully inspected wiring do not resolve the 112 code, the PCB's sensor input may have failed. A Gas Safe engineer can advise whether PCB repair or replacement is cost-effective relative to the age and condition of the boiler.
Parts you may need
- Ariston NTC return temperature probe (thermistor) · from £12
- Wiring loom / connector repair kit · from £18
- 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–£220, depending on the underlying cause.
Frequently asked questions
Is Ariston fault code 112 dangerous?
The boiler itself is not dangerous in the way a gas leak would be — it has locked out precisely to protect itself and your home. However, you should not keep resetting it and running it if the fault keeps returning, as persistent lockouts indicate a real component problem that needs attention. Call a Gas Safe registered engineer to diagnose and fix the underlying fault.
Can I fix Ariston fault code 112 myself?
The only homeowner-safe steps are checking your gas supply is on and attempting a single reset. Everything beyond that — removing the boiler casing, testing the probe with a multimeter, or replacing the sensor — must be carried out by a Gas Safe registered engineer. This is a legal requirement for work on gas appliances in the UK.
How much does it cost to fix Ariston fault code 112 in the UK?
For most households this is one of the more affordable boiler repairs. The return temperature probe is an inexpensive part (typically £5–£15 for the component), and replacement is a straightforward job. Expect to pay roughly £100–£220 all-in, covering the callout, labour, and part. Costs at the higher end of this range reflect London and South East labour rates or weekend call-outs. In the unlikely event the fault turns out to be a PCB failure, that repair will cost significantly more — a Gas Safe engineer can advise once diagnostics are complete.
Will my Ariston warranty cover fault code 112?
Ariston boilers are sold with 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 (most warranties require this), the sensor replacement may be covered at no cost. Check your warranty documentation or contact Ariston directly before booking a private engineer.
Why does fault code 112 keep coming back after a reset?
A reset clears the lockout temporarily but does not fix the underlying cause. If the return temperature probe has degraded, a wiring connection is corroded, or the PCB input channel has failed, the boiler will detect the same fault again as soon as it tries to run. Repeated resets are not a solution — book an engineer to test the probe circuit properly and replace whatever component is causing the fault.