Nothing says they have to listen but I assume some here want to learn more about how their cars work. Other than a faulty heater when dealing with an O2 code you have to ask yourself whether the sensor is bad or simply doing it's job. There are lots of problems that could occur in the system to shift the sensor signal one way or the other, even driving it full lean or rich and keeping it there. In other words a bad O2 signal doesn't mean a bad O2 sensor. It's why they're replaced so often without resolving the problem. That said, because of how the diagnostic system is programmed, in the case of code 21 the odds are the sensor or it's wiring will be at fault. With the other O2 codes the opposite is true but testing is the only way to know for sure.
