2004 Chevy Cavalier codes

Is this vehicle bi-fuel cng? Does the vehicle run ok? Have you tried some injector cleaner?

no & yes runs ok . I just put eng. in car did good then ck. eng light come on put scaner on come up with po171 code , i installed new o2 sencers , fuel filter , map sencer, gas cap also plugs. I did all this as i put in car .

There are many reasons a code P0171 would be set.
The thing is to systematically narrow down this particular reason.
If this engine has recently been installed and all the other items were replaced at the time.The suggestion by jdl123 is a good one if the recently installed engine has been setting up a while this could solve your problem. If not I would check all of the exhaust manifold bolts. A loose one could cause air to be pulled into the exhaust stream pay close attention to the exhaust to tail pipe connection. Vacuum lines are also a big headache sometime on engine exchanges. Visually inspect the O2 sensor it self as well. A pinched or otherwise damaged wire on the O2 sensor may set this code