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The other guys are right, it sounds like an O2 sensor problem or possible a header leak. They are known to get loose near the back of the engine, I usually snug up the manifold bolts a couple times a year just to be sure. It only takes a few minutes with a 9/16 wrench.
After you get done you can reset the computer by disconnnecting the battery for a minute. The drive it for a bit and see if the error comes back. If it does then I would consider the O2 sensor.
hope that helps!