Ariston 118 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 118 fault code mean?
Fault code 118 on Ariston boilers most commonly signals that the boiler's heating delivery (flow) temperature probe has returned a reading outside its expected range, prompting the boiler to shut down as a precaution. This probe — sometimes called an NTC thermistor or flow sensor — measures the temperature of hot water leaving the heat exchanger on its way to the heating circuit. When the boiler cannot get a reliable reading from it, it cannot safely regulate heat output and will lock out. A secondary, less precise interpretation occasionally cited on general advice sites links code 118 to low system water pressure, though this more accurately describes Ariston code 108. If your boiler display shows 118, focus first on the temperature probe, but also glance at your pressure gauge — if it reads below 1 bar, low pressure could be a contributing factor worth ruling out before calling an engineer.
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 (flow probe) Common
The heating delivery probe is a small electronic component that measures water temperature via electrical resistance. Over time — particularly in older or heavily used boilers — the thermistor's resistance drifts out of the acceptable range, causing the boiler's PCB to flag a fault. This is the single most frequent root cause of code 118.
- Loose, corroded, or damaged probe wiring Common
The wiring connecting the probe to the PCB can loosen at connectors, develop corrosion at terminals, or suffer minor damage over time. Even a partial break in the circuit can mimic a completely failed probe, producing the same fault code without the probe itself being defective.
- Scale or sludge build-up on the probe Sometimes
In hard-water areas, limescale can coat the sensor surface, insulating it from the water it is meant to measure. Similarly, system sludge can build up around the probe. Both reduce measurement accuracy and can cause intermittent or persistent fault 118.
- Low system water pressure Sometimes
If the system pressure has dropped below around 1 bar — due to a leak, recent radiator bleeding, or a weeping pressure relief valve — some Ariston models can produce code 118 rather than the more specific pressure-related codes. Check the pressure gauge as a quick first step.
- PCB fault Rare
If the probe and its wiring both test within normal parameters, the fault may lie with the PCB itself misinterpreting or failing to read the probe signal correctly. PCB issues are the least common cause but should be investigated once all other possibilities are eliminated.
How to fix it
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front of your boiler. It should sit between 1 and 1.5 bar when the system is cold. If it reads below 1 bar, top up the system using the filling loop until it reaches around 1.2 bar, then attempt a reset. If pressure is normal, move on to the next step.
- Reset the boiler (once or twice only) DIY safe
Locate the reset button — usually marked with a flame symbol or the word 'Reset' — and hold it for approximately 5 seconds. If the boiler restarts and runs normally, monitor it over the next day or two. If fault 118 returns promptly, stop resetting and call an engineer; repeated resets will not fix an underlying probe or wiring fault.
- Check the condensate pipe is not frozen (winter months) DIY safe
Although a frozen condensate pipe normally triggers a different code, a general lockout in cold weather is worth investigating. If the condensate pipe (usually a white plastic pipe exiting through an external wall) feels solid, carefully thaw it with warm — not boiling — water. This is unlikely to resolve fault 118 specifically but takes only moments to rule out.
- Engineer: test the heating delivery probe with a multimeter Gas Safe engineer
A Gas Safe engineer will measure the probe's electrical resistance at room temperature and at operating temperature, comparing the readings against Ariston's published resistance-temperature tables. A probe reading significantly outside the expected range confirms it needs replacing.
- Engineer: inspect the probe wiring and connectors Gas Safe engineer
Even if the probe tests within range, the engineer will check all connectors between the probe and the PCB for signs of corrosion, looseness, or heat damage. A wiring repair or connector replacement is often cheaper than a new probe and can resolve the fault completely.
- Engineer: replace the heating delivery temperature probe Gas Safe engineer
If the probe is confirmed faulty, the engineer will fit a replacement NTC thermistor. This is usually a straightforward job — the part itself is inexpensive — and the boiler should be returned to service the same day in most cases.
- Engineer: investigate and address low pressure or pressure-related components if relevant Gas Safe engineer
If low pressure was detected, the engineer will look for the underlying reason — a slow leak, a weeping pressure relief valve, or a faulty expansion vessel — and repair it. Simply topping up pressure without finding why it dropped is not a lasting solution.
- Engineer: test or replace the PCB if all other components are sound Gas Safe engineer
Should the probe, wiring, and pressure-related components all check out correctly, the engineer will assess whether the PCB is misreading the probe signal. PCB replacement is a last resort given the cost involved, so a thorough diagnostic should precede this recommendation.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If fault 118 returns after a reset and basic checks, you need a qualified engineer. Only a Gas Safe registered professional is legally permitted to work on the gas and internal components of your boiler. You can find registered engineers at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- Ariston NTC heating delivery temperature probe (thermistor) · from £18
- Probe wiring harness / connector set · from £15
- Ariston PCB (model-specific) · 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
Is Ariston fault code 118 the same as a low water pressure fault?
Not exactly. Low water pressure is more precisely associated with Ariston code 108. Code 118 is primarily a heating delivery temperature probe fault. That said, some Ariston models can display 118 when pressure falls too low, so it is always worth glancing at the pressure gauge (it should read 1–1.5 bar) and ruling that out before assuming the probe is at fault. If pressure is fine and the fault persists, the probe or its wiring is the most likely culprit.
Can I fix Ariston fault code 118 myself?
There are a couple of things you can safely do yourself: top up the system pressure via the filling loop if it has dropped below 1 bar, and attempt one or two resets. Beyond that, diagnosing and replacing the temperature probe, inspecting wiring, or assessing the PCB all require a Gas Safe registered engineer. Working on boiler internals without Gas Safe registration is illegal and potentially dangerous.
How much does it cost to fix fault code 118 on an Ariston boiler?
For the most common repair — replacing the heating delivery temperature probe including parts, labour, and a callout fee — most homeowners in the UK pay somewhere between £100 and £200. If the wiring rather than the probe is the issue, costs are similar or slightly lower. A pressure-related repair such as fixing a weeping pressure relief valve typically falls in the same range. If the fault ultimately traces back to the PCB, that component alone can cost £150–£250 or more for the part, pushing the total bill higher — worth mentioning when getting quotes so you know what a worst-case scenario looks like.
How do I prevent fault code 118 from recurring?
An annual boiler service by a Gas Safe engineer is the most effective preventive step. During a service the engineer will check probe resistance readings, inspect wiring connections, and flush any debris from around sensors before they cause a fault. If you live in a hard-water area, fitting an in-line scale reducer or a magnetic system filter (such as a Magnaclean) will help protect probes and other components from limescale and sludge build-up over time.