How do I solve my Worcester 28i boiler problem?

Worcester 28i combi boiler fault - The boiler will only fire when the flue is completely removed. I've checked the flue (horizontal type) which appears to be fine. Is a restrictor ring required for the flue which is only 1.5 metres long? New fan fitted, new pressure switch fitted, new PCB board fitted. I've also checked all rubber hoses which appear to be fine. Help!