Although working memory (WM) is amongst the most studied neurocognitive functions, temporal patterns of its component processes are not fully understood. We examined the neural underpinnings of active maintenance and interference management in the n-back task by manipulating load (1-back vs 3-back) and including so-called lure stimuli. ERPs of 27 young adults revealed that the 1-back condition enabling active maintenance showed a positive slow wave (PSW) prior to the next stimulus (−600–0 ms) and augmented P2 (190–290 ms) and P3b (330–430 ms) responses after the stimulus appeared, albeit the latter effects were driven by the initial PSW. Moreover, PSW amplitude correlated negatively with reaction time in the 1-back condition. Responses to lures showed interference, accompanied with different ERP effects for the two load levels. Our results support the view that PSW reflects efficient WM maintenance and suggest two distinct neuronal correlates for interference in WM.