Worcester Bosch 28 CDi II RSF Not Working Central Heating Light Flashing

I was asked to look at a Worcester Bosch 28 CDi II RSF combination boiler which was not working for either heating or hot water. When demand was made the boiler would sound as if it was going to light, but not and then the central heating light would begin to flash slowly.

When a tap was run the boiler would kick into action and I could hear the pump running. I opened the front cover and could see that the fan was running. I could also hear the air pressure switch clicking in and out when the fan stops and starts so assumed this was working correctly after testing for continuity with the fan running. There was a clunk coming from the gas valve so I assumed the PCB was telling the gas valve to open. I connected my manometer to the inlet nipple on the gas valve and could see the inlet pressure fluctuate as the gas valve sounded as if it was opening and closing. I checked the flue outside and diverting the products towards me could smell gas so new for sure that the gas valve was opening.

The only thing that was not occurring was sparking at the electrodes. The boiler was going through all the other sequences as it should and so I reckoned it was the PCB at fault. Thankfully I was quite convinced that this was the problem as the PCB wasn't exactly cheap even with our trade discount. The PCB that I removed from the boiler had the part number 8 748 300 394, but the part I was given at Plumb Center was 8 748 300 276 which is the same as one of the part numbers listed in the instruction book for the boiler. After replacing the PCB with the one given, everything was working well with the boiler.

Disclaimer: The works described in this blog are a record of work undertaken and should not be taken as guaranteed to solve your problem. Work should only be carried out by persons qualified and competent to do so.