optiplex 740 black screen

replace cmos battery w a new one then clear cmos settings by motherboard jumper.

now it boots, go past the dell screen, and it goes to that screen, either options of F1 or F2 goes back to the black screen

you still need to replace battery and clear cmos.  if you do not have a new, at least clear cmos now using jumper

try clear cmos settings by reset RTCRST jumper on 740 motherboard.

 
a. Locate the password and CMOS jumpers. Password (PSWD) and CMOS (RTC_RST) jumpers are on the system board 
b. Remove the password jumper plug from its pins.
c. Place the password jumper plug on the RTC_RST pins and wait approximately 5 seconds.
d. Remove the jumper plug from the RTC_RST pins and place it back on the password pins.
 
 

@redxps630​ i did it this time, but still sends me to the last screen that i uploaded, but only with the diskette drive 0 seek failure, and if still either press F1 or F2, black screen

ok time to get a new CR2032 battery.

Hi

OK;  the screen says DISKETTE, so is a floppy disk involved in the process?

It also states a fan problem, so could a fan be clogged with dust?

You could disconnect any floppy drive and see if it makes a difference.

The diskette error is very common it just means there is no FDD physically present but bios default is to look for one when not found gives error. Fan error already gone after clearing cmos

@redxps630​ yeah, all the erros are gone, excepto the diskette one, still i press F1 or F2 and bamn, black screen

the cmos is still corrupted thus you cannot enter bios or proceed to skip the error.