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A study was conducted to assess the status of the antilock brake system (ABS) malfunction warning system on in-service air-braked commercial motor vehicles (CMVs). Data from a total of approximately 1,000 CMVs were collected in California, Ohio, Pennsylvania, and Washington in August and September of 2004 by enforcement personnel who had been trained to inspect the ABS warning lamp. With four categories covering warning lamp system functionality; no lamp (including lamp could not be found), lamp inoperational (including covered up or bulb removed), lamp ON (thus indicating an active ABS system fault), or OK, a snapshot of the operational status of the ABS malfunction warning system was created for the CMV population checked. Results indicate that about one in six power units manufactured on or after March 1, 1997 showed some problem with their ABS warning lamp system. One in three trailers manufactured on or after March 1, 1998 showed a problem.