What's the cause? 14yo Trek
If by "spin" you mean that the rear hub is slipping when you pedal hard, then yes, you either need to rebuild/perform maintenance or replace the hub...there is a pawl that locks the hub while its being driven by the chain and allows the wheel to free spin while coasting. If the pawl becomes damaged or frozen in place you will experience that symptom...
It IS possible that it's just gotten rusted or gummed up with "stuff"...but it's kinda difficult to properly lubricate the internals of the hub from outside, so it will at least need to be removed from the hub to inspect, clean it up and lube.(or replace)
If that's not what you're saying, please clarify.
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I don't know what is slipping or damaged: freehub or cassette gears. Guess I'll need to disassemble for inspection -never seen inside those components.
Biggest issue is getting the thing off...you need a special tool to do it. If you know what a spline is, it looks like a long socket for a socket wrench, but the outside has a spline and the inside is smoot...
On the possibility that you don't know what a spline looks like, if you were looking into the tool, it would resemble a gear, zigzagging around a circle on the outside.
Unfortunately, they need to be purchased, so you might have to either take the bike to a shop or buy the tool if you don't have one that fits your hub.
The hub is a closed unit...maintenance usually means soaking it in gasoline, naptha, or similar, using a soft metal bristled burush to remove encrusted deposits, spinning the gears around the inner bearing, re-soaking.... finally allow it to dry and lube with an appropriate oil...I've always used 3-in-1 but trek might think differently...Actually rebuilding it is not for the novice (not saying you are, just saying )
Is this the freehub needing lubrication or will it need to be replaced?
What is it?Are you flooring it on gravel?L