Ariston 306 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 306 fault code mean?
Fault code 306 on Ariston boilers points to one of two possible issues depending on your specific model and firmware version: an internal control board (PCB) error, or a fault with an external temperature probe. The PCB interpretation is the most widely reported by UK engineers — codes 305, 306, and 307 are frequently grouped together as internal control board faults across Ariston's range. However, some models use 306 to indicate a problem with an outside air temperature sensor that the boiler relies on to modulate its heat output according to external weather conditions. Because the diagnosis differs between models, a Gas Safe registered engineer should confirm which applies to your boiler before any repair work is carried out.
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
- PCB or internal control board failure Common
The most commonly cited cause of a 306 code. The PCB is the electronic brain of the boiler, coordinating signals between all components. It can fail due to age, water ingress, or component degradation, and typically requires replacement by a Gas Safe engineer.
- Electrical supply issues causing PCB damage Common
Voltage spikes, unstable mains supply, incorrect phase/neutral wiring at the socket, or poor earthing can all damage boiler electronics and trigger internal fault codes. Interference from nearby high-voltage equipment or damaged cable insulation can also cause erratic PCB behaviour.
- Loose or corroded wiring connections to the PCB Sometimes
Over time, wiring connectors on or around the PCB can work loose or corrode, disrupting communication between components. In some cases this is a relatively straightforward fix for an engineer, without the need for a full board replacement.
- Faulty external temperature probe Sometimes
On models where 306 relates to an outside air sensor, the probe itself may have failed, or the wiring connecting it to the control board may be damaged or disconnected. Physical damage from weather or incorrect installation in a sheltered location can also cause incorrect or absent readings.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on your Ariston boiler's front panel, press and hold it for a few seconds, and wait for the boiler to restart. If the fault code clears and the boiler runs normally, monitor it closely over the next hour or two. If the 306 code returns, do not keep resetting — repeated resets will not fix an underlying electrical or component fault and may mask important diagnostic information.
- Check your mains electricity supply DIY safe
Verify that other appliances in your home are working normally and that no circuit breakers have tripped. Also confirm the socket or fused spur feeding the boiler is switched on and that the plug is properly seated. Do not open the boiler or touch any internal wiring.
- Check your boiler is still under warranty DIY safe
Ariston boilers often come with manufacturer warranties ranging from 2 to 12 years depending on the model and registration. If your boiler is within its warranty period, contact Ariston or your installer before booking an independent engineer, as a PCB fault may be covered at no cost to you.
- Contact a Gas Safe registered engineer for diagnosis Gas Safe engineer
Because fault 306 can mean either a PCB fault or an external probe fault — and the two have very different repair costs — a qualified engineer needs to run diagnostic checks on your specific model before any parts are ordered. They will inspect the control board, check wiring integrity, test the external probe if fitted, and advise whether repair or boiler replacement is the more economical route, particularly on older Ariston models.
Parts you may need
- Ariston PCB (control board) · from £280
- External air temperature sensor/probe · from £35
- PCB wiring loom / connector harness · from £45
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 fault code 306 always a PCB problem on Ariston boilers?
Not necessarily. While UK engineers most commonly associate 306 with an internal PCB or control board fault — often grouped alongside codes 305 and 307 — some Ariston models use the same code to flag a faulty external temperature probe. The probe repair is far cheaper and simpler than a PCB replacement, which is why getting a proper diagnosis before ordering parts is important.
How much does it cost to replace an Ariston PCB?
PCB replacement on an Ariston boiler typically costs £350–£600 or more once you factor in the part and labour. This is why engineers often advise owners of older Ariston boilers to weigh up whether a new boiler makes more financial sense — particularly if the boiler is out of warranty and other components are also ageing.
Can I reset my Ariston boiler to clear fault code 306?
You can try a single reset, and occasionally a transient electrical glitch will cause the code to appear and clear after a restart. However, if the 306 returns after resetting, stop resetting the boiler. Repeated resets will not cure a failing PCB or a broken probe, and they can make it harder for an engineer to gather useful diagnostic information when they attend.
Could the electrical wiring in my home be causing this fault?
Yes, it is possible. Poor earthing, incorrect phase and neutral connections at the boiler socket, or voltage instability on the mains supply can all stress boiler electronics and trigger internal fault codes. An engineer will often check the quality of the electrical supply as part of their diagnosis. If your home has known wiring issues, a qualified electrician should check the supply before the boiler is repaired, to avoid damaging a new PCB.