The major purpose of this paper is to describe that organic impurities cause harm to the arc discharge in the multiampere DuoPIGatron ion source and attempt to analyze the mechanism. Though we knew before that the existence of oil in the source is always harmful, for financial reasons, we had to use the available oil pump to pump gas. But we did not think that the oil pollution in the discharge chamber of the source could cause arc discharge failure so seriously that the source cannot operate normally. After encountering and solving the problem, we have recognized the severe action of oil pollution, and seen the damage mechanism. The up‐stream oil vapor from big pumps entered the chamber of the plasma generator. Should the parts of plasma chamber, especially, the target cathode be polluted by oil vapor, it resulted in destroying the reflected discharge in the chamber so that the stable condition of plasma, that isIig≫Iilcould not be maintained. HereIigandIilrepresent generated and lost ion current, respectively. The arc discharge died. After protecting the target cathode from oil pollution by various ways, our 15‐cm duoPIGatron can keep high performance operation for more than one year without recleaning. During this period, the arc current was larger than 300 A and the saturated ion current was above 234 mA/cm2.