I am not sure if the speed sensor is the same as the gear position sensor. But I would have someone take a look at it. I recently had problems with transmission shifting/not shifting at strange times. I took it to Volvo dealerships in two different cities and they could not figure it out (I mentioned the speed sensor after posting my problem here, but the dealerships didnt want my opinion since there were no error codes). I finally took it to an indy shop and they took a multimeter to the speed sensor and saw that it was not running with in spec(just about 20 ohms out of its tolerance). They replaced the sensor and I have had no problems since. Moral of the story; even if its not throwing any codes the sensor could still be bad. (something the dealerships should have been able to realize)