Vaillant Combi Boiler Hot Water Fluctuations Due to Flame Failure

The technical service personnel who came until today could not find the fault in the boiler. The last technical service person who came last determined that the problem in the boiler was flame failure, but could not fix the malfunction. Hot water fluctuations occur due to flame failure in the boiler. This will be my last technical service request from you, either fix the malfunction or replace this faulty boiler with a new one, this is my only request from you. I would like you to know that if my problem is not solved this time and the faulty boiler is not replaced with a new one, I will seek my rights through legal means.