1989 civic DX d15z1/Hf tranny

[nitebluesky]

Hi,
I recently swap my d15z1 motor with an Hf tranny that i rebuilt two weeks ago. Every since the weather has gotten cold my car has been throwing a code 48 and my mpgs has dropped considerably. Now its at roughly 35ish. I converted my car to obd1. My 02 sensor 5 wire wideband is brand new. Ive been scratching my head on what causes it to through a code 48 LAF?

Please help.