Background: Coping planning, the formation of plans to overcome behavioral barriers is assumed to promote health behavior maintenance, but the literature on this is inconsistent. In this study, we aimed to investigate the mechanisms of a coping planning intervention that adversely affected maintained safe water consumption. We also explored perceived behavioral difficulty as a potential moderator of coping planning interventions. Methods: In the second phase of a cluster-randomised trial, households (N = 177 analyzed) were randomly allocated to a coping planning intervention or a comparison group (repetition of interventions from first intervention phase). Safe water consumption, the mechanisms of coping planning, and perceived difficulty were measured pre-post. The data were analyzed using mediation and moderated mediation analysis. Results: Changes in behavioral intention mediated the intervention effects on behavioral maintenance (b = -0.36, 95% CI [-0.91, -0.03]). Changes in perceived coping planning (b = 0.08, 95% CI [-0.12, 0.34]), and maintenance self-efficacy (b = -0.13, 95% CI [-0.45, 0.01]) did not mediate the effects. Prior perceived difficulty moderated the coping planning intervention effects on maintenance via intention. Conclusions: Coping planning may decrease motivation for health behavior maintenance for persons who experienced few barriers prior to the planning intervention.