What can cause P0171 but o2 sensor 1 has high volts when unplugged?

I have a 2003 subaru impreza wrx. Recently it has started running really bad and the P0171 code was on along with the P0031. I replaced both 02 sensors, fuel filter, cleaned the MAF sensor. The only code showing now is P0171 which means it is running lean but the voltage of o2 sensor 1 bank 1 is really high around 3.7 V. The car runs a little better but still hesitates when trying to get up to speed. I have no idea what could be causing it.