Fans, etc run w/o 4 pin CPU plug but nothing w/it plugged in...

I'm trying to help someone with their Dell XPS 400. It wouldn't start up and the power light just stays flashing (amber). I thought it was a bad power supply so bought a new one (400w replacing the existing 375w) because nothing was running (PSU fan, case fans, optical drives, etc). Plugging in the new one into the motherboard, the PSU fan started and it seemed that was the problem. Did notice though the optical drives' lights didn't flash or open unless I disconnected the cable from the motherboard. Don't remember encountering that on other systems I've worked on - but maybe just never noticed...

Well, I got it all installed and discovered I'd forgotten the 4 pin CPU plug for the CPU. As soon as I plugged it in, I'm back to the same problem. Same amber flashing power light, nothing powers up though do notice the video card fan does "twitch" once but doesn't start. If I remove it again, the fans start up, can hear the hd start going, etc. But obviously no boot up since the CPU doesn't have power.

Reading a wide range of suggestions, I've tried clearing the CMOS, disconnecting the front panel in case USB port was bad (not an option from what I see since the the switch is controlled by the same cable), disconnecting all power cables in case it was somehow under-powered, etc. Removing all the RAM doesn't create any error beeps.

At this point I'm wondering if the MB or CPU somehow got fried. I've thought about getting another larger PSU just in case, but don't think that's going to help any. What else can I try if anything??

Thanks in advance!
3 answers Last reply
More about fans plug plugged
  1. Odds are its the motherbd. Quite common in an XPS
  2. I didn't know if should put it under CPUs or Motherboards, but figured this might be the situation.. If anyone else has any ideas to try, I'll give them a go first. Otherwise, guess he'll have to decide on what he wants to do - replace it or likely best just to get a new system.

  3. Just checking to see if any others have suggestions to rule out other causes before deciding it's the mb? Also is there any way to know it's the motherboard vs the cpu? I'd pull out the CPU and make sure it's seated well, but sure seems like Dell made it a bit tougher to do. From what can see and feel, it seems in very solidly! Of course can do a bit of searching and get the diagrams/instructions...

    To my knowledge the pc was working one day, then flashing amber power light the next time it was started up - nothing to get a loose connection (and have checked it over), etc.

    I hate to recommend and replace the motherboard only to find it's the CPU. Plus would think low cost of new desktops, age of system (performance specs, ...), etc. makes getting a new system most cost efficient?

    Thanks again!
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