Dell Optiplex 755 does not boot. The steady yellow light on the power switch comes ON but no other error codes on front panel. Machine powers up for few seconds with Yellow light and powers down then wait few seconds then the same thing happens over and over. I isolated the fault to the mother board. but do not know where to start. Replaced the processor and RAM still the same. Thanks for yor help in advance.

I have the same issue, in my case, in some Dell's I get a loud FAN noise as well, so far except for the FAN issue replacing the PSU had no effect so I think it is the MB/Proccessor. Recently dusting out the Processor worked on one of the mis-hap Dells which has thuis issue. I personally think Dell tech's should look into this and post some working base solutions!

I work at a college and had this same error with a nice few of the Optiplex 755's. However it was due to the power supply failing, nothing with the mother board. Have you tried replacing the power supply?

Is that because home power is low not every component can get the full power they needed??? maybe same problem with me

Having a similar issue. Optiplex 755 full desktop. When plugging in the power it comes on for a few seconds and then shuts down. I get diag light 3 and 4 for a second but I have tried removing both memory modules and tried them individually. Also I have replaced both the motherboard and power supply but it did not solve the issue.

Hi,

I already had same problem. I wanted replace heatsink fan. After replacement i just put the power cord to PC and the fan started to spin to maximum speed. Power buton was steady yelow and That is is all. No diag signals, no display, nothing.

Solution:

On processor heatsink I used normal thermal grease(white) instead of silver grease(silver).
My explanation is that silver grease is conductive. So it makes something between processor and cooler.

Hope this help.

It was very difficult to get the system to switch installed by the Windows installation CD or when you try to boot from a bootable CD based PEs (BartPE, Ultimate Boot CD) Dell Optiplex 755th The main culprit is the SATA hard drive and DVD players, as the drivers of these models are not available (even after a BartPE installation of Windows already exists.) The best way to solve this problem was to modify the parameters of the operation, walk in the BIOS to legacy mode, so they appear as normal IDE drives. I guess I feel the players have become a bit slower, but compromises had to be taken to install / reinstall a different version of Windows.
Dell Optiplex 755

You probably need a new motherboard. Good that you tried RAM and Processor because I was going to tell you to re-seat the processor. When a PC turns on, then after a second or two turns back off usually the culprit is a bad or mis-seated processor. If you have any PCI cards; remove them and try to power up as well.

Sounds like something is "shorting out". When your PC first powers on it is trying to check all connections and make sure everything is getting the correct "signal". If something is not working properly it will shut down to conserve it's life.

It could also be the I/O Panel. On the front of the machine it is where the actual power switch is located, it also has a thermostat in it. That can be the problem, I have seen it before.

So it's is either your Front I/O Panel or your Motherboard. I/O Panel is much cheaper. Sorry and Good luck.

Remove only jumper on motherboard. restart computer. shut down computer replace jumper. restart. This is obviously a bios reset. something in the bios become corrupt. Dell says remove jumper for 5 seconds and replace. That is incorrect. You must remove, restart, shut down, replace, restart that will fix problem. you may then have some minor configurations in the bios that may need to be fixed based on your computer config.