My speedometer works intermittently, some times it works, other times it will bounce, sometimes just won't work. Which sensor should I replaced?

 

Sensors can at times be somewhat troublesome, but just replacing a sensor before confirming it is defective or not, can get expensive .
A wise move would be to check all the wiring connections for corrosion or looseness before throwing $$$ at a possibly good sensor.

 

where may they be located.? Manuel is vague

 

Would help if we knew what year and model vehicle you are driving.

 

2004 Hyundai sonata gls v6