A novel method to measure the oxygen storage capacity (OSC) and the NOx storage capacity of lean NOx traps (LNT) has been developed. The method relies on closed loop control of the response of a HEGO sensor located after the LNT. The HEGO sensor reacts to the residual CO passing through the exhaust system during a purge of the LNT. This response can be used to infer the NOx capacity and oxygen capacity of the LNT, as well as the oxygen capacity of a TWC (three way catalyst) located before the LNT. A lean NOx trap must be periodically purged through momentary rich engine operation to liberate the chemically trapped NOx. A substantial portion of the purge fuel is also required to react with the oxygen stored in the LNT and in the TWC. During the purge of an LNT, the engine is operated rich for one or two seconds. Rich A/F operation generates a rich pulse of predominately CO that travels down the exhaust system and reacts with the TWC and LNT. The CO reacts with the stored oxygen, forming CO2. The CO also reacts with NOx stored as NO2 in the LNT, forming CO2 and N2. Unreacted CO passes out of the LNT and interacts with a HEGO sensor located after the LNT. The HEGO responds by creating a momentary voltage pulse greater than zero, corresponding to the rich A/F pulse. Capture and analysis of this pulse by the PCM (Powertrain Control Module) results in a parameter that can be used in a closed loop feedback control system. This control system can be used to measure the OSC (oxygen storage capacity) and the stored NOx. The experimental method used is described and data are presented showing the temperature dependence and magnitude of the OSC and the stored NOx. This method is currently used for off-line measurement of these two important LNT parameters but in the future can be used for on-line measurements and even for real time feedback control.