4.3L CPI oxygen sensor

I Got a 94 S-10 2wd Blazer that seems to run a little rich, like it's getting too much gas. I replaced the obvious, plugs,cap wires, ect. The engine has about 90k. I attemped to replace the oxygen senor, Even though I was not getting a service engine light. The question is, when I disconnected the orginal oxygen sensor, no light came on and it ran the same without it. According to the books I read, if the oxygen sensor is not working or disabled, you're suppose to get a service code. Could something else be wrong prior to the oxygen sensor. Any advise would be helpful.