o2 sensor reads backwards?

hello a friend asked me to figure this out his 96 grand am se 2.4l runs rich and missfires bad after warm up. he has changed the fuel pump the presure regulator, cooland temp sensor and changed the o2 sensor 3 times. befor he came to me. it had code for too lean but he cleared it at cold start computer bypasses o2 sensor with .450v and injector pulse width of 2.3 as it warms up o2 drops to .010v and pulse width increases to 4.0 when engine starts to run bad and smells like raw fuel. the engine runs ok with o2 disconnected. wiring is good from the sensor to the computer. o2 reads .018v when disconnected and wire from computer reads .348v. please help

O2 reads .018v when disconnected? voltage should be 0v when read on the sensor when disconnected. Are these voltages read with a meter on the O2 sensor? The sounds like the wiring to the computer is bad or the computer is bad.

well i relpaced the computer and the wiring is good. still doing same thing gm says mitchel is wrong .01v is lean and 1.0v is rich they said to check for cracks in the head allowing air into exhaust. but thanks for your reply