Hi casper69. As long as the new PSU is plugged in properly, then it sounds very likely that it's faulty. What does surprise me though, is that you've made such a big jump in power rating from 200W to 500W. A switched mode supply can only work over a certain power range, and I wonder if you computer that was happy with a 200W PSU has such a low demand when idling that a 500W supply falls out of regulation and switches off. If you have a multimeter you can easily check the volts on a spare drive connector. Although this isn't a complete check, if you get 5V and 12V within 10% at the drive connector than this is a good indication that the supply is working.