P0171 too lean

In the past weeks i noticed that my 2003 neon with 115000 km hesitated during acceleration and last week i got a CEL with the code P0171 system too lean. When i reseted the PCM, the car had a smooth acceleration and drove normally and then it started hesitating again and the CEL came back on. It seems to only have problems when the car has warmed up and with light to moderate acceleration. I just changed plugs and wires and it didn't help with the situation. I think it might be a bad 02 sensor but I dont know how to check these sensors...

Well thanks in advance for the help!

Check your fuel injectors as well. One or more may be plugged slightly.

The weird thing is if I reset the pcm, the hesitation stops completely and the idle becomes very smooth but the problem eventually comes back slowly, I brought it to the garage but they couldn't find anything wrong with it, i'll have to go back when the CEL comes back on