As madabouthoovers says, it is the ones which have autosave which seem to go belly-up. These are the ones which power up on a low setting, meaning that the board is regularly in use. The original hi/lo cleaners were manually locked in one position or the other, I expect typically the hi position, therefore the circuit boards were by-passed. Do you recall how many Hoover Turbopower 2-speed cleaners would always work only at one speed across both switch positions? This is because although most consumers would run the cleaner at full power, the switch was easily pressed into the "1" position, particulary if the cleaner was carried around by the main handle and not the carry handle. When the cleaner was plugged in, off it went at low speed, that was of course until the diode burned out and it ran at full power.
I agree that 2004 was the launch of the hi/lo switch. It was originally found on the back of the cleaner as there was a foot switch on top to operate the power. When the foot switch went, both controls were then mounted on the top of the machine.