Vehicle speed sensor

I replaced both input and output speed sensor along with harnesses. I am still getting a p.O 501 code. The transmission is shifting fine, the speedometer works and there is no more sputtering. My first several attempts were with boneyard parts, I was trying to get lucky. Not sure how to use this particular multimeter to test the old ones but anyhow they were replaced with brand new parts from AM auto. They cost about $36 for the pair, I added this information pursuing they are not only on quality parts. My question is what else could be causing that kickback code? Many thanks in advance.