[Vedder323]

Only one of the fans on my PSU is working while it is tripped. Is this normal? The back one is off, the bottom fan is on.

[Ridenow]

Does it have a connector that goes to a fan header on the motherboard? Some have that fan controlled by the motherboard and/or a temp sensor.

[Ange]

Could be temp controlled (or as above mobo controlled), if you cant figure out from the specs:
If it's old and not under waranty - open and check, - for a temp probe but could be difficult to find. If you can disconnect the fan from the mobo, just check the resistance of it, if its 0 or inifite the fan is dead, try to run it standalone (the fan).
If you don't wan to open it, turn the PSU on (without mobo) and blow with a hairdryer in the hottest setting trough it, if the fan works it should come on after a bit.

A.