The STPS reports the exact step out of all the steps the secondary throttle may be at to the ECU, not just open or closed. A C28 will normally be noted as the motor stuttering as it tries to position the secondary throttle plate where the ECU "tells" it to. The motor tries to step to the requested position but the stickiness or obstruction prevents that, the ECU reads the position of the secondary throttle plate, "sees" it isn't there, and signals the stepper motor to try again, etc., resulting in the perceived stuttering. After a few seconds of trying without success, the ECU throws a C28 and signals and FI.
Again, there may be other reasons for the failure than a sticky shaft like a bad STPS or a physical obstruction in the secondary throttle linkage, but the ECU is not getting a good report of the secondary throttle position for some reason.