Aids in diagnosing clogged or restricted exhaust systems. Allows comparing back pressure readings ahead and behind of suspected restriction. Ideal for diagnosing clogged catalytic converters. Kit includes small hole punch for use with air hammer, 5/16" x 1/2" self-tapping bolt and test adapter. Use self-tapping bolts to plug hole after test is completed. (Note: Do not install directly into catalytic converter or muffler.) Gauge not included. Use 0 to 15 lbs. pressure gauge.