Auxiliary power led is amber on my dell Optiplex 745 is amber and it wont boot

More than likely your PSU has died.

I don't see anything about this light, however there are 4 lights on the back panel, tell us which ones are lit - 1,2,3,4 from right to left, also, this model has diagnostic beep codes, for an example one possible beep code (code 1-3-1) consists of one beep, a burst of three beeps, and then one beep, please record when you are trying to boot up.
Provide us with lights and beep codes.

Hello !! i am with this problem right now !! can any one tell me what is the problem !! i changed the motherboard, PSU , Tried another CPU , GPU , RAM , the hard disk is working , i don't know what to do with this !! i tried every thing possible but it's still not booting and showing solid amber power light with no beeps !! and only the first light on front panel is on , BTW when i try to turn it on , the fans works but it's shows nothing on the Screen , even if the BAIOS screen , so.. can you help me plz