02 sensor gone bad?

my check engine light came on while I was driving (which means it's the emission system). I took it to friend that works at a performance shop and he hooked it up to a computer. It came up that the 1st 02 sensor...the one before the cat...was "running lean." What exactly does this mean? Does this mean it is just not reading 100%? Thanks guys.

If the computer has issued a code pertaining to the O2 sensor, the sensor and it's wiring should be tested. Usually when the sensor is bad, the engine will show some loss of power, and will not seem to respond quickly, and will drop in fuel efficiency.

This is a really helpful page:
http://www.asashop.org/autoinc/june/Tech2tech.htm

my guess is that you are having fuel issues, like clogged injectors or fuel filter. Also a list of mods will help further diagnosis..do you have any?

not many...all bolt on...AEM intake, Custom 2.5 inch piping from the cat back leading to an ANSA muffler. Here is the part that might have messed it up. I tried installing a header on my car but it fit too tightly against the oil pan, also I had to weld a nut in for the 02 sensor to screw into. This could have messed it up possibly. I have a 2000 GSR with 52,000 miles on it. I don't think I would be having problems with injectors or fuel filters yet. what do u think? One more thing...When we hooked up the computer and let it idle, the 02 sensor was reading at .000 and jumped to maybe .003 or .005 which is like not much at all.

I would substitute a known good sensor, maybe one from a friend's car.