Low_B

If you computer's fan turn on and off once you connected to the power, and the power light goes green and orange.
Sounds like the most common problem of Dell's computer.
1. motherboard
2. power supply
If you already replaced the power supply then most likely is you motherboard.
Check the capacitors of the motherboard.
If the capacitors pops up or got bubble on top, that is what causing the problem.