2000 ford explorer check engine codes

I have a 2000 ford explorer 4x4 v6 engine ligths with codes p0171 and p0174 chage the ox sensors but if i dont warmer up the engine for 15 minutes the check engine come back when i warmer up the car in the mornnirg cold temp. My check engine come and I change already the two front oxigen sensor and the check engine come back if idont warmer up the car for 15 minutes or more