PC wont power up - 4 pin ATX 12V cable when connected stops fans

After 15 months use my PC wont pwer up. I checked the power supply by shorting the green/black pins and the fan turns on. I connected the PS back to the MOBO (using 24 pin connection) and the CPU fan also turns on. As soon as i connect the 4 pin 12V supply cable none of the fans work. I have all the Hard drives, disk drive disconnected btw. Any ideas why the power is lost when the 12V/4pin connector is added?
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More about wont power cable connected stops fans
  1. Maybe bad motherboard.
  2. maybe its a bad rail, try buying a 4 pin molex adapter and see if it works off of a different rail. good luck!
  3. Without the 4 pin 12V connector the CPU has no power so BIOS cannot make it past very basic functionality like sending power to fans. Post your full system specs then go through the boot checklist, it should help solve the issue.
  4. When this happened to me replacing the motherboard brought my PC back to life. I'm going with Rolli on this.
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