The little fanless router on the shelf had been flaky for months. It would run for a week, then drop off the network with no warning, no log entry, nothing. A power cycle brought it back, so I did the lazy thing and power cycled it. Repeatedly. For longer than I'd like to admit.
Eventually the failures got close enough together that I stopped pretending it was a software problem and got the lid off.
Finding the fault
Intermittent hardware faults that respond to power cycling almost always smell of one of three things: a marginal capacitor, a thermal joint that opens up when it warms, or a connector that's worked loose. Mine turned out to be the second one, which is the most annoying of the three because it hides.
The trick with a thermal fault is to make it happen on demand. I powered the board on the bench, let it run until it dropped, then went over the suspect areas with freezer spray, a can held upside down for a quick blast of cold on one component at a time. When I hit the area around the Ethernet magnetics, the link came straight back. That narrows a whole board down to about a square centimetre.
Under a loupe, two pins on the magnetics module had that dull, slightly grey, slightly ringed look you learn to distrust. Cold joints. Probably never properly wetted at the factory, and three years of heat cycling had finished the job and cracked them clean through.
The fix
Nothing exotic here. Flux on the suspect joints, fresh 60/40 leaded solder because that's what I had and it flows beautifully, iron at around 350C, in and out quickly so the magnetics underneath don't cook. The joints went from matte and cracked to shiny and convex, which is exactly what you want to see.
symptom: link drops after ~minutes of uptime, recovers on cold boot
isolated: freezer spray on ethernet magnetics restores link instantly
cause: two cold/cracked joints on magnetics pins
fix: reflow with flux, 350C, ~2s per pin
result: 72h soak, zero drops
I put it on a soak test for three days afterwards, hammering traffic through it on a loop, and it didn't blink once. That's a board that was, for all practical purposes, dead and destined for the recycling pile.
The point isn't that I saved twenty quid, although I did. It's that a huge proportion of "dead" electronics are nothing of the sort. They've got one bad joint, one tired capacitor, one fractured trace, and the rest of the board is perfectly happy. You need a loupe, a half-decent iron, some flux, and the patience to make the fault show itself. The freezer spray is the cheat code; it turns "sometimes it breaks" into "it breaks right here, right now."
Whether it's worth your evening is a separate question, and the honest answer is usually no. But I enjoyed it, the router's back in the rack, and I learned to stop blaming the firmware for a hardware fault. That last one I'll need to relearn next month, no doubt.