Your system is detecting that the O2 is at a lean condition.
Can be caused by a faulty O2 sensor on bank 1 (near firewall, back side..) and most mechanics will jump the gun on replacing that.
Can also be caused by a lean condition going in to the engine, and the o2 is just reporting what it really does see. (vice reporting stuff not truly present)
a leak in a gasket on the manifold, or any place after the MAF really, can cause this. faulty fuel injectors could not be providing enough fuel for the air it is taking in, and making it lean.
a leak in the exhaust manifold could be allowing air to leak in just before the O2 sensor.
Check your MAF to see if it is oil fouled. Check/change your air filter. Run a few (maybe as many as three seperate tank fulls) bottles of injector cleaner through the gas tank.
Don't just jump on changing the O2 sensor, as it may not be that, and if it really IS that, then something may be underlying to ause the O2 sensor to go bad prematurely.
A little maintenance may clear it up.
Remember to reset your CEL after any changes (else it will still be lit even though you fixed the problem.)
Good luck, and let us know what you find out.
Ray
