NOx aftertreatment is an essential subsystem to enable diesel and lean gasoline engines to meet emissions regulations. A selective catalytic reduction (SCR) system, which uses urea to create ammonia (NH3) for NOx reduction, is one popular form of NOx aftertreatment system. These urea based NOx aftertreatment systems can benefit from closed-loop control when appropriate NH3, NOx, or NO2 exhaust gas sensors are available. For example, knowing exhaust NO2 emissions after a diesel oxidation catalyst can help the urea dosing strategy to maximize the efficiency of a urea SCR system. Such sensing capability, combined with ammonia sensing, can provide enhanced closed-loop control of the SCR system as well as information for on-board diagnosis.This paper covers Delphi's progress in developing an exhaust NO2 sensor. Sensor data from a synthetic gas bench and from engine testing is presented for four NO2 sensors, along with investigations into the sensor's sensitivity to a variety of relevant factors such as oxygen concentration, humidity, and operating temperature. Data regarding the sensor's cross-sensitivity to NO and NH3 are also presented, in both gas bench and engine conditions.