What are the most common underlying causes of a Potterton Boiler Error 133?

The most common causes for Error 133 include insufficient gas supply pressure or no gas, a faulty or worn ignition electrode or lead, a dirty or defective flame sensor (ionisation probe), a blocked or frozen condensate pipe, a malfunctioning fan, a faulty gas valve, or, less commonly, an issue with the Printed Circuit Board (PCB) controlling these components.