hi, i also have the same (P5V_S4RS3) MacBook Air A1466 820-00165-A 5V Rail Pulses (on & off) The strange thing is when I kept machine for 1 day and connect the power It powers on with Chime and booots half way with Apple logo and stuck. at that time when I power of and try power up Fan spins for few seconds and goes off with no P5V_S4RS3 and also the P5V_S4RS3 EN pulses on and off with 3,24 V. Plezzz doesn any one can help me