P0500 - vehicle speed sensor A and er SS on odometer

Question

My 2009 Subaru Legacy 2.5i SE, 5 speed MT, threw a cel this morning, P0500 - Vehicle Speed Sensor A. The trip odometer is showin "Er SS". I have tried resetting the code with my scantool and it will not reset. The car idles very rough and barely says running. The check engine, traction control, ABS lights are on and the cruise is blinking. Before I spend $300 on a new speed sensor, I would like to know if there is anything else I should check before buying and replacing the sensor.