After completing full 30 min diagnostic test it showed this error;

755, is 13y old.

and needs a RTC coin cell battery first, after all how fun can it be with NVRAM corrupted with each AC power pull or bump on the AC power mains to home, ? surges, lightning or even you doing wish AC power hard resets.

when THE  NVRAM goes dead, (no backup power) the RAM goes scrambled and BIOS goes NUTS>

what if the NVRAM has some DDR setting there and are now GONE. not doing service to any PC gets you there.

755 is 2007 PC

13 year old COIN cells DO NOT BELONG HERE IF TRUE. and is a first things first on all PC made, for sure.

below 2.9vdc is fail. ok?

Never diagnose Chaos from NVRAM,  (aka CMOS) a battery is $1 sold at walmart in Watch dept. 2032.

sold in vast places really.

have voltmeter this is 5 minutes work in any real shop. DMM meter, $10 even.

@savvy2 

My PC is Optiplex 755.

 https://imgur.com/a/NXhdlHW

ram fail. (your photo is still blocked but 0123 is clear)

this can be many things (even bad PSU with Chopper noise bad caps in PSU,) for sure if 13 years old,

• The RAM bad  we always remove them all and test them 1 by 1 in bank 0 ,channel A port (per the service manual)
• the also make sure the DDR are a match sets, as Intel tells US, there CPU there MMC/.ICM logic.

The secret of all PC repairs, like this is to learn the fine art of STRIP DOWN testing, see if the PC fails when LEAN.

called "a minimum system configuration"  all shops do that or go broke fast)

https://www.pcdied.com/PSU/strip-down-testing101.html

first test the PC with no USB things connected this finds shorted USB things, and also liberates more power and less noise on RAILS.

For a fact top PCs the USB is self protecting but sadly not all PCs are TOP PCs and dells do have some PCs that are not using Vbus chips at all. (not good that) Vbus chips have overload roll back and are PERFECT.

so USB is first, after all is at hand.

DO not hot swap parts inside any PC or boom.

yahhhh you have blue BIOS, just looked at youtube demo of your BIOS. (yes at bottom EVENT LOG , see that ?

yah 2,  turn on smart reporting in BIOS. page 1.  a good thing that. now .

yah3, same page data and time if any thing there is way off the COIN is dead

4: sata operations , RAID off for sure W10 , if W10 is on this PC RAID will make it sick, turn to ACHI. now.

AS seen it here.

I have no 13yr old dell here. nor with old old blue screened BIOS here.

but all mine

I power on F2 the key (hammer /spam it) and get BIOS screen 1

I then go to BIOS logs and read all those, see if other errors are there like , BATTERY DEAD) or?

I hammered the F2 key to see event log (Now Keyboard's gone). Whoa! It showed this: (how you guessed it?)

https://imgur.com/Pi4Xo2G

Its already ACHI. 

I'll try changing battery. Will it boot with all ram modules in that case?

13 years old PCs, ALL of them any maker, DT. HIT LISTs.

• COIN dead.  5-7years spec,. seen less me.
• HDD bad, fails SMART test , 13year old good HDD are rare if used everyday.
• bad PSU. caps go bad , it make noise and DDR fails or BSODS.  no PC told yet, not really.

MT, DT, SFF, USFF?

the PSU and mobo are not the same on all 755, so we ask what PC do you have of the 4.

some PC small have AC power pack the dog chewed, LOL> like the USFF has. 6 pin DC power jack rear.

Dell-chris sticky post #1 begs this,  why not tell? sorry all we can do is beg for simple facts and tell you simple tests.

@savvy2 

Bro, Is there anyway to fix a RAM Module? Because I think the problem with the faulty RAM is not a big problem or physical damage. 

the logs told you the battery is no  good ,just like all PC this old it will be bad if over  5-7 years old actual. BAT.

the coin cell  is a buck, at walmart. or more if forced to a buy pair. (i buy them in 10 packs at 25cents each)

the ram errors do not lie the log tells you truth.

you can clear the logs and start over see what come back and when does, bank on TRUTH>

one of the best things about dells is that log and the f12 ePSA diagnostics, runs even with hDD missing.

test the ram 1 by one in back 0 ,channel A.

that is what everyone does with RAM errors,  for sure, never fail to do that. if ram fails.

RAM uses shared BUS if one chip dies IT MAKES them all fail, this what unbuffered means, shared.

one more using old or USB2 cable uSB the wires inside it are super small and can not do 1 amps of current.

for sure too long,  1foot maybe but not 6feet,

use BC1.2 rated USB3 hub powered with a power pack, 2amps or more. and win.

also the Y-cable trick fails too,  due to 4-6 ports on the rear of DELL are shared (power is shared and limited to 500ma or 900mA. so adding Y-cable, is totally useless, unless you know your sharing rules like me , i have test i developed to find that, (i force the port to IOS state and see what other ports go dead, BINGO now I know on every PC have.

do not assume your USB cable good (but you tested it and is good)

I run the huge 8TB HDD 3.5" drives here and all work, but I have real USB cards in mine rated at BC1.2. 25watts.

I will test a USB HUB soon and report what works best, (I have all the tools to do that) here.

I found a top rated hub that uses VIA  VL812 Rev B2 hub chipsets , and 5amp 25watt power pack!

PLUGABLE USB 3.0 7-PORT HUB WITH 25W POWER ADAPTER #54422#

serious hardware this. more later

Hahaha.

You sure got savvy going.He will not show you his fantastic collection

And forget about the worlds fastest disk drive ... his seagate.

@savvy2 How to clear event log?

@savvy2  I changed the battery, removed the faulty Ram module, It booted up & slowed down with the time & hanged completely after 15 min (as always). Problem is different I think. BUT now it says Drive 0 not found. Its not appearing in boot menu either. Reseted BIOS but it didnt help. SATA 0 port is on. Please Help