My speedometer stopped working a month ago

My speedometer stopped working a month ago. They said they ran it through the diagnostics and that it stopped at the speed sensor. They said they ordered the part. Today I took the car in and they told me they replaced the speedometer sensor and the speedometer still does not work. And now they say they must order the speedometer for a Honda Civic EX 99. My question is how do I know the original sensor was bad in the first place. The guy told me that once the diagnostics hits the problem(the sensor) it wont go any further until that part is fixed.
Was it the right thing to do to replace the sensor. The speedometer would stay at 20 the majority of time before I got it fixed and now it is still doing the same thing. Any help would be greatly appreciated