Theory of Operation
WARNING: This page is about a different car, the 2011 RAM Pickup. However, it is still accessible from the selected car via links, so may be relevant.
The engine aftertreatment system monitors the oxygen content in the exhaust. The Powertrain Control Module (PCM) monitors the exhaust gases for oxygen content and varies the fuel injector on time to control the intake air fuel mixture. This diagnostic monitors the delta reading between the upstream and downstream O2 Sensors. The PCM will illuminate the MIL lamp when the diagnostic runs and fails in two drive cycles. The PCM will turn off the MIL lamp diagnostic runs and passes in four consecutive drive cycles.