The Power Relay System was designed to eliminate DTC 66, 67, 81, 82, and 83 on 1995-up GM trucks. These codes are many times set because the ignition switch fails to supply enough voltage to the Transmissions ignition circuit, just after start up. Once the ignition switch fails to provide adequate voltage to the solenoids, the feedback to the computer trips any combination or all of the codes above. You can replace the ignition switch, as a possible temporary fix, or install the Power Relay System and FIX the problem.