Given the cost of O2 sensors, I wouldn't recommend replacing them unless they fail. On OBD equipped vehicles, the "CHECK ENGINE" light will come on before the faulty sensor causes driveability problems. Also, keep in mind that O2 sensors can fail in several ways. Arthur Dalton is right on the money when it comes to the PO411 logic chain. When the AIR pump is switched on the O2 sensor voltage should drop below 40 mV almost immediately. As sensors age they may not react quickly enough or completely enough to satisfy the OBD test criteria. This sets a trouble code but the O2 sensor still functions well enough to ensure good engine performance. The EGR test criteria uses manifold pressure(vacuum)to determine whether or not the EGR system is working normally or not. A slight vacuum leak can cause EGR codes even though the system is working. "CHECK ENGINE" lights are really very simple to correctly diagnose as long as you know the parameters/ test criteria of the particular trouble code you're trying eliminate.