02 sensor- when do you replace it?
 

'1987 Mazda RX7 Turbo.
39k miles on it.
The car sat for the past decade.

Do you think I need to replace the O2 sensor?
Is there any way I could tell if it needs replacing?

http://www.fc3spro.com/TECH/HOWTO/ER...KI/zerror.html

First thing to do is check for error codes. It's basically stone age EFI on the s4 cars ( compared even to the s5 or FD, much less a modern OBD II system) but see if anything comes up.

Well there not that expensive, it couldnt hurt to replace anyways, especialy if it sat for a decade, unless it sat in storage, then i would think it should be fine, thats not to many miles though, i wouldnt think its bad