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First please check your auger belt. This is the most common thing to go bad. If you remove the belt cover and squeeze the auger handle you should be able to check if the belt is tight. It needs to be good and tight with minimal deflection.
On the impeller (2nd stage) there are no shear pins, but there are 2 roll pins that go through it and the axle that runs through it. These can be accessed by removing the chute and working straight down that hole.
If your gear box has an issue you should be able to grab your augers and spin them (even for a short spin) without the impeller turning.
If you have more questions please do not hesitate to respond to this question.