Hi so when I turn on my computer the blue led light on my fan turns on then fades away? I have a 700 watt power supply which is easily overkill for my machine so I don't think it's a power issue. Also my fan is still spinning. My cpu heatsink and fan is ENERMAX ETS-T40-BK Black 120mm Twister CPU Cooler with TB Vegas Blue LED PWM Fan. Thoughts on the problem?
When most computers first turn on, one of their first things they like to do is rev all of the fan headers up to full voltage (makes all of the fans spin at max for a few seconds) then after this, it ramps the fans down to normal speeds and thermally controls them.
What is possible is that the LEDs on your fan only work when the fan reaches a certain speed. This could be due to the fact that to control the fan speed, voltage to the fan is either raised or lowered.
To test this theory, you need to get your CPU hot enough where your CPU fan needs to run fast (preferably at 100%). A good way to do this is to download the free program called Prime95 and use it to stress-test your CPU (select the potion that says maximum heat). After the program runs for just a minute or two, your CPU fan speed should start to slowly go up. After about 10 minutes, it will probably be about as fast as you can get it. Take notice of your fan's LEDs at this point and if my theory is correct, they should be on fairly brightly. If they still aren't on, you may have a bad fan.
If it works, you may just have to live with your CPU fan lights being off most of the time. Not much you could do about that besides joining GIMPS and letting your CPU calculate prime numbers in it's spare time.