Agriculture is a key industrial sector in California that employs a large fraction of the workforce and contributes significantly to the state economy. The agriculture industry produces over 400 different commodities that generate over $50 billion in annual sales and over 400,000 jobs statewide. Off-road diesel engines are widely used in agricultural goods production and supply, and in 2012, approximately 14 percent of nitrogen oxide (NOx) emissions in the San Joaquin Valley (SJV) originated from farm equipment. To improve air quality and develop effective incentive strategies for the SJV, constructing an accurate agricultural emissions inventory with the latest activity data is critical. The objective of this research is to collect accurate real-world data from agricultural equipment to improve the emission inventory, and to inform policies, incentive programs and the development of future off-road engine emission standards. This research will characterize how agricultural engines in the SJV operate under actual working conditions, including their activity parameters (e.g., engine speed, torque, and fuel rate) and maintenance frequency, type, and cost. This research project will collect Engine Control Unit (ECU) data related to engine and aftertreatment performance for up to 200 agriculture tractors. The results will be analyzed to better understand the activity patterns of these tractors, which will in turn be used in the development of more accurate future emission inventories and more representative engine certification standards.