Just because you get an O2 sensor code doesn't mean the sensor has failed. It indeed may be working correctly and something else (vacuum leak, leaky injector, etc) may be causing a problem that the O2 sensor is reading correctly.
O2 sensors are delicate instruments that measure the oxygen content in the exhaust gasses. Leaded gas can kill them (no chance of that anymore) and so can the wrong silicone sealant. They operate by supplying a very small voltage to the ECM (between appx .2 volts and 1 volt). This current is totally supplied by the sensor. It does not take power from a source and modify it for the ECM. The only power source to an O2 sensor is for the heater in the sensor to help get it hot faster and get the car into closed loop sooner. (closed loop is when the O2 sensor takes over fuel management. The sensor has to be up to temp to do that) You would think that the O2 sensor would tell the ECM just how rich or lean the mixture is and the ECM would make a nice even change to the mixture to hold it at the desired air/fuel ratio. It doesn't work that way. What happens is the ECM makes changes to the injector pulse width according to the information it receives from the O2 sensor. This change will make the car go rich or lean...whatever it's intended direction was. Then, the O2 sensor immediately notifies the ECM that the car is either too rich or too lean and another set of intructions is sent to the injectors. This happens many times a second. As a result, the car sees many cycles a second of rich and lean. However, the average fuel ratio is the desired result. This switching from rich to lean is what's called "cross counts" and a good O2 sensor is capable of reading many of these per second. But, O2 sensors can just get old and tired. When that happens, the cross counts drop way down and they are incapable of making the quick change in readings necessary to the ECM. When that happens, it's time for a new sensor. A general rule of thumb is to replace them at 50,000 miles. But some may be sooner...some later. If you have a way to monitor the cross counts, you can keep your eye on them. If not, I would problaby throw a good set of sensors in the car at 50K or when needed. By the way, I would probably use OEM sensors too. Some aftermarket ones are more of a universal type design and I'm not sold on their adaptability to every car. And one thing is for sure...where the sensor deals with such minute voltages, never EVER tap into or cut and splice a wire. You're just asking for problems.
