Will a new O2 sensor help idle, power?
 

I can get anew O2 sensor for 30 u.s , and since i am putting a rb header on, i figured i would replace. do they get old, wear out, inaccurate?? my car has 270,000kms idles rough but everything else is good. is it a good idea? thanks

What year is the car?

84 gsl-se 13b efi

It can hurt gas mileage, which may cripple performance, but not likely. I would replace it if you are doing the exhaust, but I wouldn't replace it for performance gains. To make it idle smoother, maybe you need: new plugs, rotor, cap, wires, clean fuel injectors/replace. Being a 12A owner, I'm not that familiar with all the little "tricks" you can do to the 13B, but those suggestions are some basic things you might want to check out. :)

Solving GSL-SE idle problems

http://mrmazda.members.atlantic.net/gslseidle.html

Also, get a shop manual.

http://www.monmouth.com/%7Erothe/shop_manuals.html