Two questions then. (1) with the mileage on the engine, should they all be changed as O2 sensors tend to become lazy as they get older. Or at least the two downstream, Post Catalytic Convertor Sensors? (2) There is a HUGE and I mean HUGE price difference between a plug and play O.E.M. Sensor and a "Universal" Sensor. I have used Universal Sensors before and didn't have a problem, but would rather not cut and splice wires. What would you suggest? Just replace the one, two, all four..and is universal OK or go back with the original design? Thanks for your time.