I think I know the problem..thanks to my son who has a knowledge of computing sciense..his suggestion " your Power on problem has nothing to do with Power switch,especially having the same problem after a new switch fitted..micro power switch does not start the whole system,it only sends a signal to motherboard in order to start the whole system..the green light at the back of the tower and the red light inside is on so there is power available..microswitch sends the signal but motherboard does not respond, it is either faulty or there is a bad contact somewhere and this always happens first thing in the morning or after a long off period when the system is coolest..once you manage to power on the the system after several attempts, no problem with power off-on..also recently system has started shutting itself off for no apparent reason..so send it back to Dell for a replacement" which I informed the DELL
If you are "sending back to Dell for a replacement" there is no need to try and troubleshoot this unit, especially since it will work eventually.
I am not a Dell Employee
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