I'm pretty sure that's just the 'weight' that tells the 360 whether the console is upright or horizontal. Lets it know what lights to put on when you have a controller on. But yeah, it might just be the 360 itself.
E74: AV cable error... There is a problem with the AV cable, try using a different AV cable. If the cable is known to be working then there is a 90% chance it's a scaler chip problem (the "ANA" or "HANA" chip connected directly to the AV cable) in rare cases it is the GPU.