During this work effort a series of inspection and restorative maintenance actions were performed on a sample of nine 1975 through 1977 vehicles randomly selected from the St. Louis metropolitan area. Each vehicle within the test sample had been driven a minimum of 75,000 miles prior to acceptance into the program. A sequence of emission tests were performed before and after all adjustments or corrective maintenance. Such test sequences consisted of a Federal Test Procedure, a Highway Fuel Economy Test, the Federal Short Cycle, a Two Speed Idle Test and the Federal 3-Mode Test. While in their as-received condition, each vehicle was subjected to a sequence of tests followed by an inspection for engine maladjustments and intentional disablements to the emission control system. Any problems discovered during this examination were corrected and the vehicle was retested. An Emission Component Function check was performed after the initial inspection and any subsequent tests. This procedure included an evaluation of each component in the emission control system. All malfunctioning or defective components were repaired or replaced after the Emission Component Function check. Following a retest of the vehicles, each was fitted with a catalyst bypass and tested again. The bypass units were then removed and a new catalyst was installed prior to a final test sequence.