Dell Precision T5500 fails to power on and the error lights aren't in the maintenance manual.

Have you reseated all RAM modules and video card in their slots? Can you swap video cards?

Long shot:

• Power off, unplug and press/hold power button for ~15 sec.
• Carefully inspect all USB ports on the  tower to see if any of them is bent or if one of its contact is touching the case or chassis (eg, shorted out).
• If necessary, use a non-metallic item to gently fix the issue

Thanks for the advice. All the USB ports look OK, I'll dig out another video card and give that a go, I've tried it without a video to see if the thing powers up but I get the same error. 

I've tried reseating the RAM and tried different modules. Also I've tried another CPU but no luck.

Worse case scenario is that the motherboard needs replacing. The system is under warranty until May but I'm not the original owner so Dell won't play ball. I can't find out the original owner so I can't transfer the warranty. Does anyone know a way round this? 

Thanks. 

Update.....

I've just tried another video card but no luck.  

If new video card doesn't help, you may have to strip it down to bare essentials by removing all RAM except slot 1, disconnecting all drives except boot hard drive, etc.

Assuming the PSU and CPU you swapped in are working and compatible, if stripping it doesn't help, you may be right that the motherboard failed.

Dell usually requires you to have the previous owner's info in order to transfer the warranty. Do you have any proof you purchased it from the registered owner (eg, it isn't stolen)? (No insult intended.)

You may want to post in the Customer Care forum saying it's not working, should be under warranty but you need assistance transferring the warranty. Do not include the Service Tag number in that post or all the technical stuff, but you might include a link to this thread. Hopefully a Dell moderator may be able to provide some assistance with the transfer.