p0171 on warm engine when

Hello,

There are are few things that could cause this Direct Trouble code (DTC) to set. What it is saying is that your engine is running lean for some reason, and between running lean or running rich, running lean is the greater of the two evils.
I have included in the diagrams below a comprehensive diagnostic troubleshooting guide from the Ford Motor Company on this DTC and how to track down the problem and fix it. Be warned ahead of time though, it is quite lengthy as this can be a difficult DTC to track down. So please be patient and follow the guide step by step. If a step wasn't important to check out, they wouldn't have included it. You will need to have a multi-meter or DVOM to do some of the tests. I have included a link below in case you are unfamiliar with how to use one.

https://www.2carpros.com/articles/how-to-use-a-voltmeter

Please go through these guide(s) and get back to us with what you are able to find out please.

Thanks,
Alex
2CarPros