Proof-of-concept testing was conducted to evaluate the ability to identify serious losses in catalyst efficiency with a dual oxygen sensor method. The dual oxygen sensor method involves a comparison between the signal from a pre-catalyst oxygen sensor to that from a post-catalyst sensor. Testing was conducted on a dynamometer test stand in an open-loop mode under steady-state conditions. Four matched catalysts and two deteriorated in-use catalysts were tested. The matched catalysts were identical in all physical characteristics, but with varying efficiencies. The operating air-fuel ratio was dynamically varied, and the test matrix included amplitude variations of the air-fuel ratio from 0.5% to 7% above and below the stoichiometric point, oscillating at three frequencies. Results from this proof-of-concept testing show measurable differences in the pre- and post-oxygen sensor signals between catalysts with good and poor conversion efficiencies. These laboratory tests suggest that the dual oxygen sensor concept appears to be a feasible method to detect gross changes in catalyst activity with an On-Board Diagnostic system.