had to abort the smog test because of check engine light

i have a 97 dodge grand caravan, i brought it to emission with no check engine light and when i left it was on. it aborted the test due to several not ready readings and then i brought it to autozone to see what code would come up and it was the o2 sensor was reading that i need an o2 sensor, but on the emission paper states the oxygen sensor heater not ready.