Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I have a 2002 nissan altima 3.5 that has been rebuilt with many aftermarket parts. I am getting the P0171 Bank 1 lean code. I just tried to read my fuel trim info but idk what I'm doing.
LTFT bank 1 is telling you PCM is adding fuel for a long period (recorded in memory). STFT is extracting a little fuel from that memory settings (which doesn't mean the issue is fixed).
There are various causes for that (in general fuel restrictions or lack of fuel for the amount of air is entering). There are some test you must perform in order to first verify whether or not sensors are telling the truth to PCM.
Which tools do you have? Scanner, DVM, gauges, propane