Error 303

Safety first:

1) Do not attempt internal electrical work or swap boards unless you are qualified. The PCB and its terminals can carry mains voltage. Working on gas appliances requires a registered Gas Safe engineer.

2) If you smell gas, leave the property immediately and call the gas emergency number. Do not operate electrical switches.

3) Isolate mains power to the boiler at the fused isolator or consumer unit before removing any covers. If you are not comfortable doing this, stop and call a professional.

Initial homeowner checks (safe, non-invasive):

1) Note the exact error code and any accompanying symbols (e.g. spanner, lockout). Record whether code 303 is persistent or intermittent and any events before it appeared (power cut, lightning strike, recent servicing, water leak).

2) Power-cycle the boiler: switch off at the boiler isolator, wait 2–5 minutes, switch back on and observe whether the code returns. If the display now shows code 304 (too many resets) let the boiler complete any automatic reset routine; do not keep power-cycling repeatedly.

3) Check household fuses/trips and the boiler’s fused spur or RCD to ensure the unit is getting stable mains power. A tripped breaker or blown fuse could indicate a broader electrical issue.

4) Visually inspect external areas around the boiler for signs of water leaks, condensation or corrosion and check for obvious damage to wiring at accessible terminal blocks (do not remove covers to inspect wiring if you are not competent).

More detailed diagnostic steps (recommend professional for execution):

1) If the error persists after a clean power-cycle, a qualified engineer should open the unit and visually inspect the PCB for burned components, bulging capacitors, cracked solder joints, loose connectors, corrosion or signs of water/condensate ingress.

2) The engineer will check supply voltages and control signals to and from the PCB (fan, pump, gas valve, sensors) with a multimeter/oscilloscope to identify whether the fault is the board itself or an external device causing the board to report error 303.

3) Inspect and reseat connectors and wiring looms to rule out poor connections. Check communication bus links and any removable jumpers (e.g. X2 link positions referenced for other faults) for correct placement.

4) Check for related fault codes (e.g. 304 for excessive resets, 119 for missing links, fan-related codes) which can give clues whether the PCB is failing or being triggered by another component.

5) If a PCB component isreplaceable and service manuals allow, a competent service engineer may repair or replace faulty parts; otherwise the entire PCB module is replaced. After replacement the engineer will run ignition and safety checks and set any required software/configuration parameters.

When to call a professional:

1) If the error code reappears after a simple power-cycle, or if you see signs of damage, water ingress, burning or smell of burning, switch the boiler off and call a Gas Safe engineer.

2) For any live electrical tests, PCB repair or replacement, or if the boiler is locked out and will not resume safe operation, always use a qualified engineer.

What to tell the engineer:

Provide the boiler model, exact error code (303) and whether it was accompanied by other codes or a spanner symbol, describe any recent power interruptions, visible damage, flooding or rodent activity, and what troubleshooting steps you already tried (power-cycle, fuse checks, photos if available). This information speeds diagnosis and helps the engineer bring the correct parts and tools.

Note: Do not attempt to bypass safety interlocks or operate the boiler if it remains faulted. PCB faults affect safety-critical controls and require professional diagnosis and repair.