Ariston 302 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 302 fault code mean?
Fault code 302 on an Ariston boiler signals an internal communication error — the main printed circuit board (PCB) has lost the ability to exchange data correctly with another board or electronic component inside the appliance. This is a 300-series code, which places it firmly in the electronic control and PCB category rather than a sensor fault. It is worth noting that the flow temperature sensor has its own separate code (110), and the return probe has code 112 — code 302 is not a sensor reading fault but rather a breakdown in the electronic conversation happening between internal components.
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 damaged wiring between PCB and components Common
Vibration over time or poor connections from a previous repair can cause wiring harnesses to work loose inside the boiler. When the electrical signal between the main PCB and a secondary board or component is interrupted, the boiler logs a communication fault and shuts down. This is the most frequently confirmed cause when an engineer investigates a 302 code.
- PCB failure Common
The main PCB acts as the boiler's central nervous system, coordinating every electronic function. If the board itself develops a fault — whether through age, moisture ingress, or a power surge — it can no longer maintain reliable communication and triggers code 302. A failed PCB requires professional diagnosis and, in most cases, replacement.
- Software or firmware fault on the PCB Sometimes
In some instances the PCB's onboard software can become corrupted or fall out of date, causing it to mishandle communication protocols with other modules. An engineer may be able to update or re-flash the firmware rather than replace the board outright.
- Sludge or scale causing erratic component behaviour Sometimes
Heavy sludge or limescale build-up inside the heat exchanger or pipework can cause components to operate outside their normal parameters, which in turn generates irregular electrical signals that confuse the PCB and trigger a communication fault. This is more likely in hard-water areas or systems that have never been power-flushed.
- Neglected or overdue servicing Sometimes
Electronic faults in the 300-series are more commonly found on boilers that have not been serviced regularly. Annual checks catch deteriorating connections and component wear before they escalate into lockout faults.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Ariston boiler's control panel and hold it down for up to 10 seconds until the boiler attempts to restart. If the 302 code clears and the boiler runs normally, monitor it closely over the next hour. If the fault returns, do not keep resetting — repeated resets will not cure an underlying electronic fault and may mask useful diagnostic information for the engineer.
- Check the boiler's power supply DIY safe
Confirm that the boiler's isolator switch is on and that the relevant circuit breaker or fuse in your consumer unit has not tripped. A momentary power fluctuation can occasionally trigger a communication error that a single reset resolves permanently. If the breaker keeps tripping, leave it off and call an engineer.
- Do not attempt to open the boiler casing or touch internal wiring Gas Safe engineer
Code 302 originates inside the boiler's electronics. Inspecting or adjusting internal wiring, boards, or connectors must only be carried out by a Gas Safe registered engineer. Opening the casing yourself risks electric shock, voiding any remaining warranty, and is not permitted under UK gas safety regulations.
- Engineer to inspect all internal wiring and connectors Gas Safe engineer
A Gas Safe engineer will remove the boiler casing and methodically check every wiring harness and connector linked to the PCB and any secondary control boards. Loose, corroded, or burnt terminals are re-seated or replaced. This step alone resolves a significant proportion of 302 faults at relatively low cost.
- Engineer to assess PCB condition and update firmware if applicable Gas Safe engineer
If connections are sound, the engineer will test the PCB itself. In some cases a firmware update can resolve a communication protocol issue without replacing the board. If the board has failed — evidenced by burnt components, moisture damage, or failed diagnostics — a replacement PCB will be sourced and fitted.
- Consider a system power flush if sludge is suspected Gas Safe engineer
Where the engineer suspects that sludge or scale is contributing to erratic component behaviour, a power flush of the central heating system, followed by the addition of inhibitor, is recommended. This is carried out by a qualified heating engineer and helps protect the new or repaired PCB from future stress.
- Call a Gas Safe registered engineer if the fault persists after one reset Gas Safe engineer
If the 302 code returns after a single reset attempt, book a Gas Safe registered engineer to carry out a full diagnostic inspection. You can verify an engineer's registration at GasSafeRegister.co.uk. Do not continue to run or reset the boiler repeatedly, as an unresolved electronic fault can develop into more extensive damage.
Parts you may need
- Ariston main PCB (model-specific) · from £220
- Wiring harness / loom · from £45
- PCB connector repair kit / terminals · from £15
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 £120–£400, depending on the underlying cause.
Frequently asked questions
Is Ariston fault code 302 dangerous?
Code 302 is an electronic communication fault rather than a gas or combustion safety issue, but the boiler will lock out and stop working to protect itself. You should not attempt to bypass the lockout or repeatedly reset the boiler. Book a Gas Safe engineer to diagnose it — leaving an electronic fault unresolved can lead to more expensive damage over time.
Can I fix Ariston fault code 302 myself?
The only homeowner steps worth trying are a single reset and checking that the boiler has a stable power supply. Everything beyond that — opening the casing, inspecting wiring, testing the PCB — must be done by a Gas Safe registered engineer. Working on a gas appliance's electronics without registration is illegal in the UK and unsafe.
How much does it cost to fix a 302 error on an Ariston boiler?
If the fault is a loose wiring connection, you are likely looking at an engineer call-out and labour cost of around £120–£200. A PCB replacement is significantly more expensive — typically £300–£500 or more once parts and labour are included — and on older boilers it can make financial sense to obtain a new boiler quote at the same time. The figures in the repair-cost range above reflect the most common scenario (wiring or minor PCB work); a full PCB swap sits toward or beyond the upper end of that range.
How do I prevent code 302 from coming back?
The single most effective prevention is an annual boiler service by a Gas Safe registered engineer. Regular servicing catches deteriorating connections, cleans components, and keeps the boiler's electronics in good condition before minor wear triggers a lockout. In hard-water areas, ensuring the system has adequate inhibitor also reduces the risk of sludge-related stress on the PCB.