Real-world nitrogen oxides (NO x) emissions were estimated using on-board sensor readings from 72 heavy-duty diesel vehicles (HDDVs) equipped with a Selective Catalytic Reduction (SCR) system in California. The results showed that there were large differences between in-use and certification NO x emissions, with 12 HDDVs emitting more than three times the standard during hot-running and idling operations in the real world. The overall NO x conversion efficiencies of the SCR system on many vehicles were well below the 90% threshold that is expected for an efficient SCR system, even when the SCR system was above the optimum operating temperature threshold of 250 °C. This could potentially be associated with SCR catalyst deterioration on some engines. The Not-to-Exceed (NTE) requirements currently used by the heavy-duty in-use compliance program were evaluated using on-board NO x sensor data. Valid NTE events covered only 4.2-16.4% of the engine operation and 6.6-34.6% of the estimated NO x emissions. This work shows that low cost on-board NO x sensors are a convenient tool to monitor in-use NO x emissions in real-time, evaluate the SCR system performance, and identify vehicle operating modes with high NO x emissions. This information can inform certification and compliance programs to ensure low in-use NO x emissions.