32 | P a g e
303 Description: PCB Error
Cause: The PCB has a software or hardware error.
1. Check boiler polarity and voltages are correct.
2. Check boiler is properly earthed using an earth loop impedance tester.
3. Check all connectors on the PCB for loose or broken contacts. Replace as required.
4. If code 303 remains, replace the PCB.
304 Description: Reset too often.
Cause: The reset button has been pressed too many times to clear a fault, more than 5 times in 15 minutes.
1. Wait 15 minutes and reset the boiler.
2. Switch off the boiler from the fused spur, wait 5 seconds and re-establish the electricity supply.
3. If this does not help after a couple of attempts, replace the PCB.
306 Description: PCB defective
Cause: PCB defective.
1. Check boiler polarity and voltages are correct.
2. Check boiler is properly earthed using an earth loop impedance tester.
3. Check all connectors on the PCB for loose or broken contacts. Replace as required.
4. If code 306 remains, replace the PCB.
309 Description: Gas valve relay control error
Cause: Flame signal detected after the gas valve has been de-energized.
1. Check whether the boiler is properly earthed using an earth loop impedance tester.
2. Check the ionisation pin condition and the ionisation cable for a short circuit. Replace as required.
3. Check if the flame remains after the demand has ended and if there is still voltage on the gas valve. If so,
replace the PCB.
4. If no voltage at the gas valve and the flame remains, replace the gas valve.
5. Check all wiring and connectors to the gas valve and PCB for loose contacts and breaks and check continuity
of wires using a multimeter. Replace as required.
6. Check the gas valve and PCB connectors for traces of moisture and corrosion, replace as required. Determine
where the moisture comes from and fix the issue. Potential causes: POC recirculation, leaking air vent, etc.
7. If intermittent and all other checks OK, replace the PCB.
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