Recently had the issue on a XPS 8700 where pressing power button wouldn't turn on PC. PSU was working, as confirmed by pressing button on back of case. I could get PC to power on by unplugging everything (not just power cord), holding power button down for 20 seconds, plugging everything back in, and then pressing power button normally.
After doing this routine a few times, I replaced the CMOS battery and PC started up on first try. When I measured the voltage of the old battery (original, and probably several years old), it shows 3.1V. So maybe the issue wasn't the battery? It was definitely properly seated.
Subsequently, the issue has reappeared, so it's definitely not the battery.
I also recently picked up a XPS 8900, and it has the same intermittent non-responsive power button. I used the same procedure to get it to start - unplugging everything, not just power cord), depress power button for 20sec, plug everything back in and then press power button.
Seems a bit too coincidental, unless Dell has a massive failure rate of several year old XPS power buttons? Both PCs were plugged into the same UPS. I plugged the 8900 into the wall and experienced the same failure.
Any ideas on what might be causing this?
Your problem sounds very similar to that discussed in this thread: https://www.dell.com/community/XPS-Desktops/XPS-8700-power-button-does-not-work/td-p/5808419 Perhaps the discussion and solution can help you.
I removed the top cover to access the power button module on both machines. Sprayed them both with electronic contact cleaner. That seems to have done the trick for the 8700. I power cycled it 4x in a row and it powered up each time on the first press. The 8900 powered up the first time after the cleaner, but not after that. Unplugged, spray a little more, and once dried, it powered right up. But subsequent attempts were not successful. So I left the machine powered off with the power module dangling off the top side of the case, and was doing some browsing on the 8700, and maybe 10 minutes after leaving the 8900 alone, it powered up. Strange.
I'm pretty sure it's the power button module. The test button on the PSU powers up the PSU and the fans. PSU also powers up for a couple of seconds when plugging in power cord. So I think the PSU can be ruled out. Once the machine powers up, it behaves normally, so I think that probably rules out the motherboard.
New switches are available on ebay, such as this one: https://www.ebay.com/itm/Power-Button-Assembly-Cable-for-Dell-XPS-8900-7R574-07R574/282913096413
Here's a closeup of the switch. For anyone who has replaced one of these, how do you non-destructively remove the old switch from the plastic power button housing? I can see the clips holding in the lights, but it's not clear to me how to remove the old switch.
Some other sites have the entire assembly, under the same part number, but considerably more expensive: https://www.impactcomputers.com/7r574.html