The main source of defects under multi-pass arc welding with a consumable electrode in a protective atmosphere are the fluctuations in the spatial movement of the arc during transverse oscillations of the electrode in the groove. The research problem of the influence of divergence of electrode oscillation parameters on the formation of lack of fusion during multi-pass welding of pipelines was solved by computer engineering analysis. The use of modern research methods made it possible to reasonably establish the relationship between the parameters of the arc welding process with transverse oscillations of the consumable electrode, which ensure a defect-free weld formation. It was determined that the surface of the filling pass does not have any significant convexifications and cavities if the span of lateral oscillations covers the entire width of the groove at the level of the surface of the previous pass, and the interval period and welding speed ensure the formation of a weld pool, the width of which is equal to the width of the groove at the indicated level. However, when the center of transverse oscillations is displaced from the plane of symmetry of the joint, the surface of the filling layer is distorted and a significant difference in the filling levels of the groove occurs, which in the limit reaches the thickness of the bead being deposited. This results in transverse cavities on one side of the groove and longitudinal cavities on the opposite side, which are the places where lack of fusion is likely to occur. It was also found that the subsequent layers equalize the filling level of the groove, however, if the penetration depth during surfacing of the next layer is less than the size of cavities in the previous layer, a high probability of the formation of lack of fusion appears.