If the blower is running without being all the way up I would check that the motor/gear box has stripped. First make sure your filters are all the way in and locked. They have switches behind them. Remove screws securing the blower and the venting. This will give you access to the lift motor. If the gears have slip it will hit the blower switch. Check that the actuator has not slipped. Removing the whole unit is the best and easiest way to replace the lift motor.
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Thank you. Let me know what you find.