Try replace cmos battery with a new battery and clear cmos settings via motherboard RTCRST jumper.

Core 2 quad support may depend on the 755 SFF motherboard Dell P/N.

I did change the cmos battery already but did not clear the rtcrst jumper so it is something i can try. Would going back to bios a20 be worth while? You would imagine a22 would have all the same attributes as a20 with some fixes but as i said in my orginal post some have found a20 will only work for Quad CPUs. 

thanks for the reply

I highly doubt Dell would remove quad support in latest bios while keep it in a slightly older ver bios.  If after cmos clear still no post it would suggest this motherboard revision is not a quad board.  unfortunately Dell does not release info on which board (DP/N) has quad support.

This motherboard (DP/N= PU052 rev A01) has quad support

another way to find out is to type in your 755 service tag on Dell support site and view product spec. For example this 755SFF is validated by Dell to run Q6600.

223-6833 : OptiPlex 755 Small Form Factor Core 2 Quad Q6600/2.40GHz,2X4M 1066FSB

 
vs this one core 2 duo

223-0606 : OptiPlex 755 Small Form Factor Core 2 Duo E6750/2.66GHz,4M,VT 1333FSB

there is a subtle difference between the chassis used for quad vs duo processor. The finding of v2 chassis for quad processor suggests it has a rev2.0 motherboard, which then suggests rev1.0 mobo may not have quad support.

otoh, this 755 core 2 vPro has quad support

223-7156 : OptiPlex 755 Small Form Factor Core 2 Duo E8400/3.0GHz,6M,VT 1333FSB

Thanks very much for the reply. Honestly didn't know that it depended on which motherboard you had wether Quads would run or not. 

If it is the case and my board won't run a Quad i guess a duo at 3.0ghz wouldn't be a bad alternative.

Hopefully today i can try clearing the CMOS and see if that helps and also investigate which board i actually have in my 755 sff. 

Cheers

That's the 760 in which it can get a little confusing if it will run a quad or not.

Except for the USFF, 755 can take up to a Q9650.  It's not in Dell doctrine.  Some years ago, I ran into some experts in this forum that lead me to the Q9650 and it worked great in my 755 MT and has worked for others.

A22 BIOS should be fine.  Definitely try the CMOS reset redxps suggested.

On 2nd thought, I may have heard of the 755 not taking a Q8400.  Dell didn't indoctrinate it for the 755.  Q9650's are still quite reasonable.

Optiplex 755 Tech Guide 

Q8400 is validated by user benchmark in 755.

I realise the CMOS reset would set all settings to defaults but out of interest how would that enable the CPU to suddenly work?? Totally not denying it could work but just wondering the logic behind it. 

I will report back when i do the reset. Thanks for the replies, much appreciated. 

Thanks for the reply. That makes sense. 

bios rom or bin file flashed in cmos has a cpuid table to look up supported cpu microcode during POST.  
in case the table or cmos settings in ROM memory is partially obscured in an old pc,  clear cmos may help.

@redxps630, thanks for the info on the Q8400.

Just an update to this problem. I reset the CMOS by using the Jumper Method and also checked the battery which is fine.

The results from 3 different Q8400 that i tried are as follows

CPU 1 Orange Light on Constant, nothing on screen

CPU 2 Green Power Light on but Bios keeps rebooting and fans run very high.

CPU 3 Green Light on but 3 and 4 errors lights on with nothing on Monitor.

After all this testing i replaced with the Pentium 1.8ghz and i got the 3 4 lights error again but if i hold the power button until the pc shuts down and then switch back on the pc boots fine. 

Anything else i can try or is the just destined to not run a Quad?? 

Cheers

I'd try a different quad.  You know me by now - I'd try the Q9650.  I'd definitely have a hard time tolerating a 1.8ghz Pentium.

If it still won't take quad, may be time for a newer PC?

I had a Q9650 and an SSD in my previous 755 with Win10 and the thing flew.

May be with CPU 3, try each RAM stick one at a time and see if it makes a difference, or may be with Pentium in it.

Sorry to see it's giving you so much trouble, I'm a bit surprised.

If all 3 Q8400's were defective, that would be quite the coincidence.

When using the second Quad 8400 i was able to enter the BIOS and it has been detected. Just the constant rebooting the issue. 

I do have another Q6600 arriving but it will probably come after Christmas at this stage. 

Just to add i tested all 3 Q8400 i have in a 780 sff and they all worked fine.

I got a 7010 recently but just want to give this 755 a new lease of life if at all possible.

I ran the Diagnostic Utility and got this message which would suggest no boot drive is detected but when i hit f12 my ssd is listed. The fans run super fast and you would think its going to take flight lol.