I replaced a video card fan in a Compaq desk system.

Now it will not boot.

Power is getting to the power supply.

And I changed out the switch on the front cover, but still nothing.

What else can I check ?

No. Separate topic.

One tip in troubleshooting is always go back to the last thing you changed. Have you verified that any connections disturbed are reconnected correctly? If you disconnect the fan does the computer power up again.

It is possible you damaged or shorted something out on the graphics adapter when the fan was installed. I would pull the graphics adapter out and see if the computer will boot. You can tell if it is working by the CPU and case fan spinning as well as the BIOS power on beep (assuming it has a speaker). Some BIOS will halt if it detects a graphics card failure.

It boots if it has no graphics card.

Sounds like my card went dead.

Just found out that computer boots with card installed but fan disconnected.

Must be a bad or incompatible fan yet it is supposed to be the correct replacement.

I will find out what voltage it runs at and see if the runs by itself.

I requested that the seller allow me to return the fan.

I guess I will have to buy another PCI Express card.

I got it working.

After cleaning up thermal paste that had extended beyond the GPU chip, it started working. :-)

I am guessing that the paste in electrically conductive.