Forester won't move from first gear

My wife is describing an *intrermittent* problem on her manual 2002 Forester that she also links to the car being left very cold (it's low negative
temps overnight here):
Occasionally, the car will not move from the first gear, behaving as if it is in neutral. In all such cases she experienced (three so far, with me not being able to reproduce them), she was able to move from the 2nd gear.
It has the original clutch with 126,000 miles, mostly in city driving. Idling RPM seems normal.
Any ideas what it might be and/or how to test/diagnoze it further?
Thanks much,
Dima
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DK wrote:

Had an old manual car that would freeze up on extremely cold days. One day I went with my aunt to a hardware/lumbar store. Couldn't get the car out of gear (to change gear) upon leaving the store about an hour later. Paid someone $20 to take us home. Dad came over and we went to the store with a tow strap figuring on having to tow the car to a nearby car shop. Voila, the car would shift. Apparently it had warmed up enough either when we left the store and had the engine running but spent around half an hour trying to figure out why I couldn't disengage the clutch, or the temps were up the next day when Dad and I returned to the car.
I had a Jeep that had a cable from the clutch pedal to the clutch housing. When that snapped, obviously I couldn't change gears. That's an example of a mechanical clutch. Some cars use hydraulic assisted clutching: uses brake fluid to facilitate clutch operation. There is a reservoir for the hydraulic clutch fluid aka transmission oil. The hydraulic clutch fluid reservoir is probably close to the brake fluid reservoir (but some systems share the same brake fluid reservoir for brakes and hydraulic clutch). The hydraulic clutch fluid is DOT 3 or 4 brake fluid. Brake fluid is hygroscropic: it attracts and absorbs water. That's why you eventually have to do a brake fluid flush (along with it turning brown from contamination). Since the brake fluid used for the hydraulic clutch fluid absorbs water is why it could freeze and make the clutch not operate.
The clutch fluid level (if a separate reservoir from the brake fluid reservoir) can get low during normal usage. There are numerous symptoms when clutch fluid is low (won't bother listing them). Just remove the cap and check the fluid level (unless you can clean the outside of the translucent housing to note the fluid level). There are high and low marks for the fluid level.
https://www.manualslib.com/manual/232308/Subaru-2002-Forester.html?page39
That shows the clutch fluid (transmission fluid) reservoir is separate from the brake fluid reservoir. Remove the cap to check to clarity of the fluid. Should be clear or golden, not brown. If the cap is leaking in outside air, could be the fluid has been contaminated with moisture hence the freezing up.
Subaru claims the clutch fluid never needs to get replaced but does recommend check the fluid level is between Min and Max. However, since it is brake fluid and because brake fluid is hygroscopic then the fluid could be contaminated with water. I suspect the system is considered "sealed" so air shouldn't be getting to the fluid to get its water content absorbed by the fluid, but leaks occur and cap seals go bad.
First just check the clutch fluid reservoir's level is above Min. Then test for sticky shifting on very cold starts and immediate use of the car after starting. Of course, immediately using your car that way under extreme cold is bad for other components, like the power steering. In fact, as part of my extreme cold start, and after the car has ran a minute or two, I turn the wheels a little bit to get the power steering fluid flowing around before cranking it to full stop on reversing out of my winding driveway to get through a fence gate. Not smart to start and immediately drive in extreme cold. Fluids don't move as fast when extremely cold, and absorbed water changes the characteristics.
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