Do both sensors on a 1999 mazda miata have to be changed when the code says to replace oxygen sensor?

The last three auto shops have only replaced the sensor after the CAT and not the one by the manifold|||I would replace both, although the guys are right. The front one is the important one|||The code itself specifies the sensor that has failed. I just replaced a rear sensor on my 2002 with a Bosch 15733 generic.|||Get the header checked out. These do crack and cause a fault reading of the O2 senors. The down comer (from the header to the cat) can also develop a crack and the same thing happens.|||Typically the one after the CAT never needs to be replaced. 99% of the time it is the front o2 sensor that needs to be replaced since that is the one that the computer needs to adjust fuel.