Urban elevated-road exits concentrate deceleration, merging and surface-road control within a short transition, yet their emission responses are difficult to resolve from routine vehicle records. This study analyzed six dates of 1 Hz GPS and on-board diagnostics from one China VI diesel commercial vehicle operating in Nanjing. Sequence map matching linked the trajectories to a frozen OpenStreetMap network and identified 92 stable layer 1 → 0 exit occurrences. Leave-one-date-out telemetry models estimated the downstream OBD-derived NOx indicator expected from recorded vehicle, engine and aftertreatment states without using road variables. The trip-balanced post-minus-pre residual contrast was 0.084 log units (95% confidence interval, 0.020–0.151), whereas same-trip non-exit controls balanced on recorded operating changes showed a smaller contrast. The direction persisted across alternative models, map-matching settings, timing falsification, continuity thresholds, low-speed inclusion and an analysis restricted to recurring sites. Twenty sites were observed on multiple dates. These findings identify a recurrent, map-aligned OBD-derived NOx response signature within the observed operation and provide a practical basis for prioritizing targeted field measurements of emissions, traffic and road geometry at complex urban transitions.