2001 Kia Rio Check Engine light is on

Dear Sir/Madam,

My check engine light came out 1 month ago. The code was P0501 which is the speed sensor. My mechanic changed the vehicle speed sensor but the code came out. After I went to the Kia dealership and they said that the wheel speed sensor is the problem. My mechanic changed it. Code on again. I went back to the Kia. They replaced the front axel bc they said it was broken. Code back. They replaced the transmission speed sensor. Code on. They checked the ECM and the wiring. Everything is fine but the code is on. The computer cannot be reprogrammed bc it was done once in the past on the behalf of the previous owner.

They still have no idea what can be the problem. I would really appreciate if anybody could help what can cause the P0501 code instead of the speed sensor.

Thanks,

Laszlo