Booshy here. 2008 Scion xB battery is good, starter was taken out and checked good, negative ground was cleaned on the frame side still no start. Recently put new tranny in was driveable to a certain extent throwing codes po741. But was still drivable. One day went and got gas filled up the tank came back out turn the key no start. When you're turning the key on sometimes you get a click at the starter sometimes you don't after you get one click it won't click again for awhile. Like maybe the the next day. Sometimes when you turn the key on all the lights on the dash come on and the battery light kind of flickers sometimes it's non-existent every once in awhile it will just flicker flicker flicker flicker then it'll go away and disappear 2 minutes and come back flicker,flicker. Any ideas. I was thinking maybe take the old neutral safety switch from old transmission maybe switch it. Any other ideas maybe starter relay I don't know where it is lol if there is one. This is just a out of the blue problem never have any problems all starting vehicle. Any suggestions would be helpful.

 

Sound more like you have a power or ground issue for the battery. You'll need to check the connections with a multi-meter to make sure you are getting full voltage from the battery to the starter and to the main fusible links in the engine compartment fuse box.