Question Computer fans spin for a second then nothing

[AspectSaber]

So I recently upgraded my office computer's power supply, and now when I put all of the cables back, the computer fans (CPU, GPU, and exhaust fan) spin for a second when I hit the power button, but stop and nothing happens. However the power supply light is on. Pressing the power button again does nothing, until I unplug the power cord and put it back, which just makes the fans spin for a second. I've been stuck with this for a month and I'm getting real tired. Things I have done was completely replacing the CMOS battery, changing the RAM slot, and testing the power supplies with the paperclip thing and they both work (old and new). I have completely taken apart the computer but can't find the problem. The power supply is a 400 watt which should easily power the i3 7100 and gt 1030 in it. Please help.

 

[AspectSaber]

Hey guys I know it’s been almost a year and to be honest I have just kept this thing not doing anything for a whole year and today I started to get into reviving it again but to no avail, is there anything else I can try before I completely recycle this thing?

 

[Paperdoc]

Hmmm. Thread seems to be missing some earlier posts, so we'll start "from scratch".

What you describe appears to be brief attempt at start-up followed immediately by a complete shut-down. I can think of four things that can cause such.

1. On virtually all mobos the CPU_FAN header does THREE jobs. It provides power to the CPU cooler, it adjusts that cooler device's speed, and it monitors that device's speed signal for NO signal (or in some cases, a speed too low) that indicates failure. Failure detection means that the mobo will shut down the system completely right away, without waiting for the temperature sensor inside the CPU chip to show high temps. This is to prevent very rapid heating of the expensive CPU leading to permanent damage from NO cooling. So, check whatever device you have for the CPU cooling (fan and heatsink, or pump of a AIO liquid-cooled system). WHERE is that plugged in? IF it is NOT in the CPU_FAN header, move it to there. IF it is plugged in there, UNplug it, then plug back in again. Repeat a few times. Then CHECK that it is secure AND that you did not disturb any other things during this time. This action may "scrub" any dirty surface on the contacts in the connector so it makes good contact again. Now try to start up. Does that particular fan start even briefly? Or does it do nothing? Or (hopefully) it runs properly.

2. On most systems there is a multi-contact cable from the PSU directly to a socket on the mobo very near the CPU ship that is power for that chip alone. This is in addition to the largest PSU output cable going to the mobo. Without that connected securely it cannot start up.

3. In making your changes (replacing the PSU plus ???) did you remove the mobo from the case? Do you know for sure that the system worked properly BEFORE the PSU replacement? I'm thinking about mis-location of the stand-offs under the mobo that hold the mobo away from the backing plate of the case. A stand-off in the wrong spot can cause a short circuit from a mobo trace on the bottom side to the case. But if you never had that problem before, it is possible but not very likely that this is your problem. If you need more info on this, post back here.

4. It is possible your new PSU is faulty. Can you replace it temporarily with a known-good PSU to check? I once had a PSU that failed slowly over a period of many months. A main symptom was that the system would crash completely and not re-start until I disconnected from the wall outlet for 15 min, then plugged back it. It would start and run for a while, then fail again.