What is P0500 code on nissan sentra 1996

Question

I'm told it's the speed sensor. The speedometer is working but the odometer is not.

Answer 1

The speedometer head is probably the problem.  Before replacing the speedometer head, try retightening the screws behind the speedometer. The speedometer connects to the circuit board with screws, if the screws are loose it may triggered the P0500 code.