In your step 3, when LED stayed on beyond 3 sec, was it solid or flickering? Is the PSU fan spinning when you do this step, and does it continue to spin? If the LED continues to flicker in step 3, I'd suspect an ailing PSU.  A compatible PSU is maybe ~$50-$60+. So you have to decide if you're willing to spend that amount, and if you can find one that can be returned for a full refund, if it doesn't solve the problem. Just make sure a replacement PSU has the same or higher wattage as the one installed now...
Hi, I have a Dell Inspiron 560 which does not power on. The only sign of life is a flickering light on the power supply. It is not a steady blink, and the power cord is good. Is it possible that this means it has a bad power supply? Thank you for your time.
Flickering Light on Power Supply