99 xk8. What steps to take to find the problem, if anyone has had that trouble code.

 

Use P0171 and then you can use Search to see what to do.

 

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I have attached the definition for the P0171 code you are receiving. It also lists possible causes. I would first check the air inlet pipe bottom side for cracks in the convoluted area. A pretty common problem.
The most likely cause of the code is a vacuum leak allowing un-metered air into the system.