The smoking PC: lessons in dust and fans

“Stef, can you fix my PC please?”

I looked up from my desk. That question is a common one, and I’m happy to help out because it’s what I do. I immediately posed my standard question to ascertain what flavour of help was required: usually driver installation, printer issue, network troubleshooting, yahde yahde. So imagine my surprise when the response was:

“The computer’s on fire.”

Quite true, it was. At least, acrid smoke was billowing profusely from its interior, and the office stank of burning plastic. Once the power source had been removed and it had cooled sufficiently I checked the interior. Here’s what I found:

As you can see, something had clearly gone wrong. On closer inspection, over and above the layer of soot coating the motherboard, there was a lot of dust in the machine. But dust alone doesn’t normally catch fire, even with the heat inside a typical computer case. There must have been another catalyst, and I wanted to know what it was.

Enter PC Sleuth

The computer was a pre-built system from some computer firm or other and, while it didn’t have components I’d have chosen if it were my own machine, we’re not talking PC World kind of awful. ASRock motherboard; Athlon II X2 265 (3.3GHz) CPU which isn’t renowned as a particularly hot core; a single 160GB hard drive; DVD burner; 4GB of DDR-3 RAM; on-board graphics; wireless card… in other words, nothing out of the ordinary.

I started disassembling it, checking things out. Did the usual power supply test outside the case, shorting pins 15 and 16, and it was fine aside from the fan being a bit noisy (probably a bearing going, thanks to the dust). Made a mental note to replace it. The CPU heatsink and fan were intact. The SATA power connector to the drive looked to be damaged as that appeared to have been the epicentre judging from the state of the power cable. But aside from a few sporadic forum posts, I’d never heard of a hard drive setting fire so didn’t expect anything untoward. Removing the controller board on the drive proved that, and judicial scraping / hacking at the carbonised contacts revealed the gleaming metal beneath. At least the data was recoverable.

Out came the motherboard, I cleaned everything, blew the copious quantities of dust away, looked for any signs of what might have caused the burning. Nothing, nada, zip. Just charred cabling leading to the PC’s front panel that had been touching the ill-fated molex-to-SATA power cable (an extender). The spur from the power supply to which it was connected was fine, but clearly something must have triggered such massive heat in that end portion of the cable.

Scratching my head, I kept working backwards, stripping cards out, cleaning, poking, testing, inspecting, until it was the turn of the furthest component away from the damage zone: the case fan. I blew hard to clean the layers of dust, expecting the fins to turn in the process. They didn’t. In fact, the blades would hardly budge. The thing was seized firm, and I mean firm as if the centre was soaked in Marmite.

At least I had a possible culprit. It wasn’t attached to a motherboard header so the BIOS hadn’t been informed of the fan’s treacly status and therefore didn’t initiate its thermal shutdown. But it still didn’t make sense. Even if the case fan failed, the machine had plenty of airflow and, as I mentioned, isn’t a particularly hot piece of kit at the best of times.

My only possible prognosis — and this is based on my rather shaky electronics knowledge so may be way off base — is that the load put upon the power supply to try and drive the stuck fan motor caused a massive current surge through the spur. Such a strain could have backed up into the (rather cheap-looking) PSU and instead of causing it to cut out, resulted in a tonne of heat coursing through the wire which just melted the poor SATA cable at the end of the line.

As I say, it’s uncertain as to the exact cause because I’m not that smart: if anybody with greater knowledge than me has any clues, please let me know your ideas. But three important things did come out of the experience:

1. Clean out dust regularly before it clogs fans.
2. Connect fans to the motherboard wherever possible so they can be monitored.
3. Don’t use a cheap power supply.

All of these things I knew already (though I’m guilty of not cleaning computers often enough), but it seemed the manufacturer of the machine didn’t consider the dangers points (2) and (3) pose when combined with massive negligence of (1).

Lesson learned: clean more regularly. I’m just glad that someone was in the office when it happened; if it had occurred in the left-switched-on machine overnight we might have been greeted next day by a pile of ash where the building once stood.