Modern light-duty vehicles require well-controlled engine-out feed-gas and very high catalyst efficiencies to meet the US Environmental Protection Agency (EPA) Tier 2 & 3 standards. When a vehicle with either a gasoline or diesel engine is operating within its controlled state-space the exhaust emissions present at the tailpipe are extremely low. When it is not operating within its controlled state-space the combustion process and therefore its exhaust emissions characteristics will be different. This may occur when an emission control device fails or if a defeat device is employed. Moreover, different control technologies each have unique characteristics or signatures that could assist in identifying either emission control device failure or an existing defeat device.A simple exhaust extension apparatus equipped with a thermocouple for measuring exhaust temperature and a NOx / O2 sensor to measure tailpipe NOx and O2 concentrations can characterize this signature information for pattern recognition analysis. This device can be used both in a laboratory environment with conventional batch sampling systems or for on-road testing as a compact emission measurement system.If this information was acquired during conventional laboratory emissions tests it would provide valuable dynamic system information. This information could characterize events such as cold start open loop operation, engine transient fuel compensation, and high-speed load enrichment and emission control device status with minimal cost.