no first gear in automatic
 

my 1999 will not start out in low gear. I have to manually put it in low and then it only goes to 2nd. shifts to 3rd and 4th. what did I need to check to find the issue. thanks...trans has been rebuilt just before I got the car but has had the issue ever since Ive had it. just got the ls1 rebuilt to 383,

No first gear, may be a bad shift solenoid. However if the trans was rebuilt you may want to check and make sure it's actually the correct one for your car. Some transmissions are only suitable for their exact year/ make/ model.

replaced shift solenoids still no low gear, scanner said it is commanding 1st gear. any help would be greatly appreciated

Well that's not the good news we were looking for. From here the options really narrow because you have the command for first gear. Let's try the only simple one first.
you need a comprehensive wiring schematic for the trans. You will need to verify that the signal for 1st gear is getting all the way to the trans. Sometimes the wires just fail.
Saying you have signal at the trans, your next options become the valve body or a bad band or clutches.
There could possibly be other avenues to look at but we're getting above my pay grade here. Good luck and keep us posted.