3040 MT uses Dell proprietary 8 pin ATX. The PSU is TFX form factor and has the 8 pin connector.  DP/N  240W  THRJK 4GTN5 4R1KT D7GX8 H62JR 3RK5T 6WX7D HGRMH 2P1RD 0M1C3 J1J77 RKTF0 Alternatively you could get a 24-to-8 pin adapter for 3020/3040 with a standard 24 pin ATX psu.  [URL] If you have an old working pc you could borrow its 24 pin psu and get the adapter above to test your optiplex 3040, before committing to psu replacement.
Hi, I have a 4-year old Optiplex 3040 and for the last 3 days I had to press the power switch several times before it would light up and power up the system. The first 10-20 times I pressed it nothing happens. Fortunately after several tries the light comes up and the system powers up just fine. Do you think the power switch is getting ready to fail? Can I buy only a power switch or do I need a new power supply? Or it could be something else that's wrong? Maybe the CMOS battery? Thank you.
The power switch of my Optiplex 3040 needs to be pressed several times to power 
up the system