In support of the regulations prohibiting the sale of devices which bypass or defeat automotive emissions control strategies, a study was undertaken to compare emissions generated from vehicles using original (OEM) and aftermarket programmable-read-only-memory (APROM) chips. Six different vehicles and chips from three separate APROM manufacturers were tested in this investigation. The driving cycles used in this work were the standard Federal Test Procedure (FTP) driving schedule and a cycle that closely resembled the California acceleration (CA) cycle. The test plan was based on running two OEM emissions tests, followed by two APROM tests, and then a final OEM test. The data were analyzed using an unpaired t-test to compare differences between means. Data for one vehicle was analyzed using a two way analysis of variance to separate driver effects from PROM effects. This was necessary due to the use of two different drivers during testing.The APROMs and vehicles have not been specifically identified since this data is still viewed as confidential. However, only one APROM was used per test vehicle so that identification of the vehicle will serve as the APROM identification also. On specific vehicles there was strong statistical evidence (> 95% significance level) for increases in FTP HC up to 27%, FTP CO +59%, FTP NOx +53%, FTP CH4 +17%, and fuel economy decreases of roughly 2%. Generally speaking, all vehicles appeared quite similar when driven on the CA cycle., although CO and NOx was increased 93% and 176% respectively in specific cases. Please note that it is possible to obtain large percent differences when measuring small values such as exhaust emissions. Also, other differences in FTP and CA fuel economy and emissions were seen at lower significance levels.