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@Dramatic_Hammer as hugo7 said, when the motor stops working. Faults with the 1200watt henry are the on/off switch wearing out, the circuitry for the hi low motor and the moulded plug cable breaks at the plug cable joint so cut the old plug off and install a new one. The retraction contacts can wear out too.
Friend, who has long hair swears by Kirby, second hand tho not new as they are eye wateringly expensive and you can't buy them the normal way. Rebuildable and all parts are available even on old models but they come from the states so the post and packing cost are high.
When it stops working.
It's a repair to one of the common faults rather than a preventative / maintenance point.
But the main thing is that many of the repairs on a Henry are DIY and although it requires specialist parts, most of those parts are cheap.