Mine did that when I installed the shift kit...Some dumbass forgot to hook up the pressure solenoid(me). You might want to drop the pan and check your connections.
Dave
2003 Silverado SS here's where my money went...Radix @ 9psi, Vortec IC tank, Oversized Heat exchanger, ASM LT Headers, Suncoast Ram Air hood, Airaid intake, Trailblazer Torque converter, Full billet servos, Zippy kit, cat-free, Flowmaster muffler, Gaylord Speedstur tonneau, 3/4 drop with Belltech spindles and leafs, Sirius conversion & MTX thunderform sub box. 05 e-fans coming soon!