Innovation through modular product design is a promising strategy for product lifetime extension and material recyclability. Yet, sustainability benefits of the design do not automatically come into effect, but require additional process innovation through service activities that have to be attractive and made use of by users. This study investigates to what extent sustainability-focused modular product design in the smartphone sector promotes (1) self-repair versus use of repair service and (2) a positive user experience with repair instructions and services. It further analyzes how circular economy attitudes and perceived self-repairability moderate these effects. The quantitative analysis finds that, strengthened by higher perceived self-repairability, users of modular smartphones are more likely to use repair instructions. Also, modular smartphone design increases positive experience with repair instructions. Consequently, successful implementation and management of complementary product and service innovation are key to promoting product lifetime extension in the smartphone industry.