d15z1 problems in a 92 civic lx

[j21857]

so this weekend i swapped my new motor into my 92 civic lx 5 speed.its a d15z1.i am only getting about 225-250 miles per tank of gas which is terrible.i have two cel codes coming up code 12 and code 48. i cant tell if the o2 sensor is working or not, i know i dont have an egr box. does anyone know where to stat as far as finding out why the gas mileage is so terrible?