Theory Of Operation

WARNING: This page is about a different car, the 2015 RAM 1500. However, it is still accessible from the selected car via links, so may be relevant.

The intent of this diagnostic is to identify rich or lean fuel system abnormalities which would cause vehicle emissions to exceed 1.5 times any of the standards. This is accomplished by monitoring the Short Term and Long Term fuel trims. If they exceed a calibrated threshold for a calibrated time, a fuel system problem is occurring. This feature is active under all closed loop fuel conditions.

Since the Downstream O2 Sensor is after the catalyst, it is protected from chemical poisons (like silicon and phosphorous) and protected from thermal aging. The Downstream O2 Sensor feedback optimizes the catalyst efficiency and normalizes the tailpipe emissions based on the Downstream O2 Sensor output, especially when the system components are no longer at nominal values. The Powertrain Control Module (PCM) monitors the Downstream O2 Sensor feedback control, called downstream fuel trim, to detect any shift in the Upstream O2 Sensor target voltage from nominal target voltage.

Upstream and Downstream fuel trim monitors operate very differently. The Upstream O2 Sensor can adjust the goal voltage to accommodate for slight fuel/air variances. The Downstream O2 Sensor has a fixed goal voltage to maintain catalyst efficiency.

The upstream fuel trim diagnostic is designed to detect a large change (33% from nominal) and will generally fail quickly.
The downstream fuel trim diagnostic is designed to catch small failures (from worn components) over a longer period and therefore takes much longer to fail.
The possible causes for both diagnostics are the same but the upstream is detecting a gross failure condition and the downstream will detect a smaller, less obvious condition.