The Z420 power supply is turned on when pin 8 is driven low by the motherboard circuitry. If this does not happen, the power supply will not turn on.
P1 pin 17 is an auxiliary +11V rail, meaning this rail is active whenever the AC power cord is plugged into the system. If there is no voltage here, the "turn on" circuitry in the Z420 is not powered so the system will not start.
Edit: An ATX power supply puts +5VSB (standby = auxiliary) power on pin 9. The Z420 needs a 11V auxilary voltage. If this +5VSB is connected to P1 pin 17, it might not be high enough to turn the system on. (This is an educated guess, I have not tried using an ATX supply on a Z420). Be careful, connectors are always numbered differently, and sometimes pinouts are shown as a mirror image (I have to look at the keyed/empty pins to get the orientation right)