Ariston 303 Fault Code: Causes, Fixes & Repair Costs
What does the Ariston 303 fault code mean?
Fault code 303 on an Ariston boiler indicates an internal fault with the main Printed Circuit Board (PCB) — the electronic control unit that manages virtually every function of your boiler. This is part of Ariston's 300-series codes, which as a group relate to the boiler's main PCB and electronic control systems. When this code appears, the boiler may become unresponsive to controls or lock out entirely. Note: a small number of online sources incorrectly describe 303 as a return temperature sensor fault — that is wrong. The return sensor fault on Ariston boilers is code 112. Code 303 is definitively a main PCB fault, as confirmed by Ariston's own published fault code tables.
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
- Internal PCB component failure Common
The PCB itself has developed an internal fault — a circuit, relay, or onboard component has failed. This is the most common outcome when code 303 appears and typically requires the board to be replaced. PCBs on older boilers are particularly susceptible as components degrade over time.
- Power surge or electrical short circuit Common
A sudden voltage spike — from a lightning strike, a power cut restoration, or a fault elsewhere in the electrical system — can damage the sensitive electronics on the PCB. Burnt or scorched tracks on the board are a telltale sign.
- Loose or corroded wiring connections to the PCB Sometimes
Vibration over time can cause wiring connectors to work loose from the board. In some cases this is a straightforward fix for an engineer without needing a new PCB. Corrosion on terminals can produce a similar intermittent fault.
- Water or condensation ingress Sometimes
Boilers installed in lofts, garages, or other exposed locations in the UK can suffer from condensation or drips reaching the PCB enclosure. Moisture on electronics causes short circuits and corrosion that can trigger code 303.
- General age-related degradation Rare
On boilers more than 10 years old, capacitors and other PCB components can simply reach the end of their service life, causing the board to produce erratic signals and trigger internal fault detection.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on your Ariston boiler (usually marked with a flame or reset symbol) and hold it for a few seconds. Be aware that Ariston boilers allow only around 5 resets within a 15-minute window before a deeper lockout can occur, so do not keep repeatedly pressing reset. If the boiler fires up and runs normally, monitor it closely. If code 303 returns, move to the next step.
- Check your household electricity supply DIY safe
Confirm the boiler's fused spur is switched on and that no fuses have blown or circuit breakers tripped in your consumer unit. A momentary power interruption can sometimes cause a one-off PCB error that clears on reset.
- Power-cycle the boiler DIY safe
Switch the boiler off at the fused spur or unplug it, wait at least five minutes, then restore power. This allows capacitors to fully discharge and gives the PCB a complete restart. This is worth trying once before calling an engineer, but it is not a long-term fix if the fault returns.
- Do not attempt to inspect or work on the PCB yourself Gas Safe engineer
The PCB is a mains-voltage component and working on it without the correct training and equipment is dangerous. Do not open the boiler casing, disconnect wiring, or attempt to dry out or repair the board yourself. This work must be carried out by a Gas Safe registered engineer.
- Call a Gas Safe registered engineer to diagnose and repair Gas Safe engineer
A qualified engineer will open the boiler safely, inspect wiring connections for looseness or corrosion, and test the PCB. If the fault is a loose connector it can often be resolved quickly at low cost. If the board itself has failed, the engineer will advise on PCB replacement. Given that a replacement PCB can cost upwards of £400–£500 including labour, on an older boiler it is worth asking the engineer for an honest assessment of whether a new boiler would be more cost-effective.
Parts you may need
- Ariston main PCB (control board) · from £280
- PCB wiring loom / connector set · from £35
- Boiler fused spur switch · from £12
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–£450, depending on the underlying cause.
Frequently asked questions
Is code 303 really a PCB fault, or have I seen it described as a sensor fault elsewhere?
Code 303 is definitively a main PCB fault according to Ariston's own published fault code tables. Some lower-quality websites — including a number of AI-generated pages — incorrectly describe it as a return temperature sensor fault. That is wrong. If your Ariston boiler has a return temperature sensor problem, it will display code 112, not 303.
Can I fix Ariston fault code 303 myself?
The only homeowner action you should attempt is a single reset and a power-cycle (switching off at the fused spur for five minutes). Both are safe and occasionally clear a one-off electronic glitch. Beyond that, the PCB operates at mains voltage and all further diagnosis and repair must be carried out by a Gas Safe registered engineer. Do not open the boiler casing or disconnect any wiring.
How much does it cost to fix an Ariston 303 error?
If the fault turns out to be a loose wiring connector, an engineer can often fix it in a single visit for around £100–£150 (call-out plus labour). If the PCB itself needs replacing, expect to pay roughly £350–£500 or more including parts and labour — PCBs are one of the most expensive boiler components. On a boiler over 10 years old, it is worth getting a new boiler quote alongside the repair quote, as a full replacement may be better value in the long run.
What other fault codes are related to PCB problems on Ariston boilers?
Ariston's 300-series codes are all linked to the main PCB and electronic control systems. Codes 305, 306, and 307 in particular are also considered serious PCB-related faults where replacement of the board is commonly needed. If you see any of these codes, contact a Gas Safe registered engineer promptly rather than repeatedly resetting the boiler.