Processor Upgrades

You have optiplex 755 DT board. Per tech guide Q6600, Q6700, and E6850 and below, E4600 and below are compatible w 755 in general.  Dell-Chris M said for 755 in general

These are the Quads Dell validated -
N652J 3.00GHz Yorkfield Quad Core Q9650, 12MB
F886J 2.83GHz Yorkfield Quad Core Q9550 12MB, 1333FSB, E0 
N650J 2.66GHz Yorkfield Quad Core Q9400, 6MB, 1333FSB, R0
MP627 2.66GHz Kentsfield Quad Core Q6700, 8MB, 1066FSB, G0

E8400 is not on Dell list but it is also compatible proven by you.

your board may not have latest bios to support Q6600.

according to this eBay listing DR845 supports quad 2.4 which is q6600.

Yes ebay post has the same motherboard and processor as I have but I don't understand why isn't the processor working with my motherboard?

My BIOS are A22 dated 6/11/2012. It's the latest version which I downloaded from Dell's official website.

But still Q6600 doesn't work. I installed the processor Q6600 in the motherboard but it doesn't pick it up. It starts for 2,3 seconds and then shuts down. No display screen appears either i.e the loading screen. It just keep getting restarted like it's trying to pick up the processor but it seems like the motherboard is not compatible with Q6600.

Early I also tried to install Intel Core 2 Quad Q9400 but the same thing happened. No display and loading screen and just the system getting restarted over and over again.

I could not find any solution nor any website or forum to know what processor does my board support. Can you help me out find it please?

Did you upgrade to latest bios?

can you verify the Q6600 works in another pc?

Thanks. I found the answer. The problem was with the bus speed. My PC had 1333MHz while the CPU was of 1066MHz. Now I installed Q8400, it has 1333MHz and it works fine with my PC.

Thanks for your help. Really appreciate it.