ATAG 303 Fault Code: Causes, Fixes & Repair Costs
What does the ATAG 303 fault code mean?
Fault code 303 on an Atag boiler signals a problem with the PCB (Printed Circuit Board) — the central control unit that acts as the boiler's brain. Every component in the boiler communicates through the PCB, so when it develops a fault, the boiler cannot operate safely and will lock out. The code can be triggered by a genuinely failed PCB, but also by wiring faults, moisture ingress, earthing problems, or issues with flame detection circuitry that feed signals into the board — so a Gas Safe engineer will investigate systematically before condemning the PCB itself.
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
- Moisture or corrosion on the PCB or its connectors Common
Water ingress from an internal leak — for example from a leaking auto air vent, a weeping heat exchanger, or condensate products — can track across PCB contacts or corrode connector pins. This is one of the most frequent root causes engineers find when investigating code 303.
- Loose or damaged wiring to the PCB or gas valve Common
Vibration over time can loosen plug-in connectors, and cable insulation can chafe against metalwork, causing intermittent shorts or open circuits that the PCB misinterprets as an internal fault. Engineers check every harness connector and test for continuity before assuming the board itself has failed.
- Earthing fault Sometimes
If the boiler is not properly earthed — or if the earth connection has deteriorated — the PCB can behave erratically, trigger spurious faults, or suffer latent damage. Engineers use an earth loop impedance tester to verify the installation earth is sound.
- Ionisation or flame detection circuit fault Sometimes
The ionisation pin and its cable provide the PCB with proof-of-flame feedback. A short circuit in this cable, a contaminated or damaged ionisation electrode, or a flame that persists after the gas valve closes can all produce signals the PCB cannot reconcile, triggering code 303.
- Power surge damage Rare
A voltage spike on the mains supply — from a nearby lightning strike or a grid disturbance — can damage sensitive PCB components. This tends to cause immediate and permanent failure rather than an intermittent fault.
- Age-related PCB failure Rare
Electronic components degrade over time. Capacitors dry out, solder joints develop micro-cracks, and processor components can simply reach end of life — particularly on boilers that have been in service for ten or more years.
How to fix it
- Check that the boiler's gas supply is on and that other gas appliances in the property work DIY safe
Make sure the gas isolation valve on the boiler's gas supply pipe is fully open (handle in line with the pipe), and confirm that a gas hob or other appliance lights normally. If you have no gas at all, contact your gas supplier before going further.
- Reset the boiler once or twice using the reset button on the control panel DIY safe
Press and hold the reset button as described in your Atag user guide (usually 3–5 seconds) then allow the boiler up to two minutes to restart. If the fault clears and the boiler runs normally, monitor it closely. If code 303 returns within a short time, do not keep resetting — repeated resets will not fix an underlying fault and can mask useful diagnostic information for the engineer.
- Check system pressure and top up via the filling loop if below 1 bar DIY safe
Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, use the filling loop (two valves, usually under the boiler) to top up slowly until the needle reaches 1.2 bar, then close both valves. Low pressure is unlikely to cause code 303 directly, but it is worth ruling out before the engineer arrives.
- Check for any visible water leaks around the boiler DIY safe
Shine a torch around the boiler casing, pipework connections, and beneath the unit. If you can see dripping water or damp marks, note the location and let the engineer know — moisture tracking onto the PCB is a leading cause of this fault code.
- A Gas Safe engineer should inspect all wiring, connectors, and the gas valve Gas Safe engineer
The engineer will power down the boiler and methodically check every harness connector to the PCB and gas valve for corrosion, loose pins, or broken wires, testing continuity where needed. This step alone resolves the fault in many cases without any parts being replaced.
- A Gas Safe engineer should test the boiler's earth continuity Gas Safe engineer
Using an earth loop impedance tester, the engineer verifies that the boiler is properly earthed per current wiring regulations. A poor earth can cause persistent electronic faults and must be corrected before the PCB is condemned.
- A Gas Safe engineer should inspect the ionisation electrode, pin, and cable Gas Safe engineer
The engineer will check the ionisation pin for carbon deposits or physical damage, inspect the HT lead for shorts to earth, and confirm the flame signal is behaving correctly. A faulty ionisation circuit can generate the signals that trigger code 303 without the PCB itself being at fault.
- A Gas Safe engineer should identify and repair any source of moisture ingress Gas Safe engineer
If corrosion or moisture is found on the PCB or its connectors, the engineer traces where the water is coming from — common culprits include a leaking auto air vent or heat exchanger — and makes that repair first. Fitting a new PCB into a wet environment without fixing the root cause will result in the replacement board failing too.
- If all other checks are clear, a Gas Safe engineer should replace the PCB Gas Safe engineer
Atag PCBs (such as the LMU-B control unit used in several i-Series models) are specialist parts that often need to be ordered directly from Atag or an authorised distributor. The engineer will fit and commission the new board and verify the boiler operates correctly across a full heating and hot water cycle before leaving.
- If the boiler is within its 10-year Atag warranty, contact Atag directly before authorising any repairs DIY safe
Atag offers a 10-year standard warranty on their boilers and uses their own engineer network for warranty repairs. Call Atag on 0800 680 0100 or contact the Atag Selected Installer who originally fitted the boiler. Authorising independent repairs on a boiler still under warranty could affect your warranty entitlement.
Parts you may need
- Atag PCB / LMU-B Control Unit · from £480
- Ionisation electrode · from £35
- Ionisation (HT) cable · from £22
- Wiring harness · from £65
- Auto air vent (if found to be leaking) · from £18
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 £450–£700, depending on the underlying cause.
Frequently asked questions
Can I fix Atag fault code 303 myself by resetting the boiler?
A single reset is always worth trying — occasionally a transient electrical glitch will clear after a reboot. However, code 303 specifically points to the PCB control circuit, and if the fault returns after one or two resets you will need a Gas Safe registered engineer. Working on a boiler's PCB, gas valve wiring, or internal components is not DIY territory, both for safety reasons and because incorrect work could void your warranty or home insurance.
How much does it cost to replace the PCB on an Atag boiler in the UK?
Most homeowners pay in the region of £450–£700 all-in for a PCB replacement on an Atag boiler in 2025, with the part itself (such as the LMU-B control unit) accounting for £430–£530 of that and the remainder being engineer labour. If the engineer's investigation reveals the fault was actually a wiring or earthing issue rather than a failed board, the cost can be considerably lower. In rare cases where additional components have been damaged, costs can rise beyond £700 — which leads many people to weigh up a new boiler, particularly if the existing one is older or out of warranty.
Is it worth repairing an Atag boiler with a PCB fault or should I get a new boiler?
Atag boilers are premium units, so if yours is relatively young, a PCB repair is usually worth doing. A common rule of thumb is: if the repair costs more than half the price of an equivalent new boiler installation, replacement deserves serious consideration. For an older Atag boiler out of warranty, a £500+ PCB bill — on top of potential other age-related repairs — can tip the balance towards a new boiler. Your engineer can give you an honest view once they have diagnosed exactly what has failed.
My Atag boiler is still under warranty — who should I call for fault code 303?
Atag provides a 10-year standard warranty and handles warranty claims through their own approved engineer network. Call Atag directly on 0800 680 0100 or get in touch with the Atag Selected Installer who originally installed your boiler. Do not authorise an independent engineer to replace the PCB before speaking to Atag, as unauthorised parts or labour can invalidate your warranty cover.
Could code 303 be caused by something other than the PCB itself?
Yes, and experienced engineers always investigate this first. The code indicates a PCB fault signal, but that signal can be generated by a short circuit in the ionisation cable, corroded connector pins, a poor earth connection, or moisture from an internal leak tracking across the board's contacts. In a meaningful proportion of cases the PCB is actually fine and the fix is a new ionisation electrode, a repaired connector, or sealing a small leak — all far cheaper than a new control unit.