the O2 sensor reads the exhaust, however it comes out, and tells ecm how to compensate, add or subtract gas. if your ignition system isn't up to par, you will get funny readings as the sensor(s) are trying to tell the ecm what it reads. fouled or worn plugs, bad wires and such will not burn the air/fuel mixture properly, thus your exhaust gas will not come out at an optimum, which will cause sensor to read funny as it tries to help adjust. make sure all ignition and fuel delivery is good, then move on to sensors.
heated sensors will read open when cold. they need to be heated to temp before they actually start reading.
rednekper4mince. keepin duct tape and bailing wire in business.