Off-chip serial buses are common in embedded systems, and due to the long physical lines, can contribute significantly to their energy consumption. However, these buses are often connected to analog sensors, whose data is inherently affected by noise and A/D errors. Thus, communication can tolerate small approximations, without a significant impact on the system outputs quality. In this paper we propose an energy-efficient approximate serial encoding for sensors data, inspired by the T0 technique for parallel buses. Despite its small encoder/decoder overheads, this method is capable of significantly reducing dynamic power (> 90%) in off-chip serial lines, with negligible average error (<1%) on decoded data.