PC Frequently Rebooting due to Power Supply Unit Issue

This past Saturday, I was having lunch with the family in the kitchen when I heard from the office the sound effect that my new PC plays after booting.  I thought that was odd (since no one was in there using the PC), but shrugged and figured that maybe the PC had rebooted itself due to some automatic update?  When I heard the same sound again a few minutes later, though – indicating a second reboot in the span of a few minutes, with no one sitting at the machine – I investigated.

As I worked on the PC, I found that it was rebooting itself every few minutes, at irregular intervals.  The PC had never done this before; aside from some heat-related bluescreen issues after I first put the PC together, the PC had been quite stable for the 6 months or so since it was built.  I had the reboot problem reproduce once while I was in the boot-time BIOS configuration utility, which proved the problem wasn’t a Windows issue.

Due to the intermittent nature of the problem, I initially suspected it might be caused by another overheating issue.  However, the BIOS configuration utility showed that the CPU was operating at a nice and cool (relatively speaking!) 40 degrees C or so. 

I also suspected a possible RAM issue – perhaps one of the four RAM modules had gone bad and was causing the reboots.  However, I tried removing two of the RAM modules at a time (with the machine powered off, of course); the spontaneous reboot reproduced with only modules #1 and 2 in the machine, and also with only modules 3 and 4 in the machine, which appeared to rule out a memory module issue.

Finally, though, I got “lucky” in my investigation.  After the machine rebooted itself twice in rapid succession, I manually toggled off the power switch at the back of the PC’s power supply unit (PSU) to keep the machine off while I (once again) manually inspected the machine’s hardware.  When I toggled the power supply unit switch back on, I got a surprise – a bright spark and a loud “POP” sound from within the power supply unit!  I immediately pulled the power cord out of the PC to prevent anything further from occurring with the PSU.

This seemed unlikely to be a coincidence.  If the PSU had indeed gone bad somehow, I could see that the behavior of the system rebooting (with no error message, except Windows complaining that it hadn’t been shut down properly at restart time) would be consistent with the system losing power momentarily, and then getting it back immediately afterwards.  I was fairly confident that the incoming power supply to the machine was OK, as I had the machine hooked up to power via a UPS (an uninterruptable power supply -- designed to ensure a consistent flow of power, even in the event of a power failure).

I replaced the apparently faulty power supply unit with a spare that my father-in-law Doug had on hand (thanks Dad K.!) – and that resolved the issue!  The machine has been running since Saturday afternoon with no problems.  So the intermittent spontaneous reboot problem had been caused by a faulty power supply unit. 

I wrote “BAD” on the faulty PSU and threw it away.  I also ordered a modern 500W PSU with integrated PCI-Express and 8-pin CPU connectors to replace the one I’m borrowing from my father-in-law.

The lesson learned here for me is not to cheap out on buying a PSU when building a new PC!  When selecting a case and power supply for this machine, I’d mostly paid attention to cases, and had just accepted the included power supply that came with the case – for the PSU, I had only really looked at the raw wattage on the PSU (450W).  The PSU had turned out to be an older/cheaper model with no PCIe connection, and only one SCSI drive connection.  For this PC, with reasonably demanding power requirements (including a GeForce 9800 PCIe video card), the lower-end power supply that came with the case apparently just wasn’t sufficient over the long term.

In the future when building PCs, I’ll most likely opt for purchasing the case and power supply unit separately, and will certainly pay more attention to the detailed specs of the PSU to ensure that it is likely to be sufficient for the needs of the system I’m building!