How many Oxygen Sensors on 1998 5.7 L Vortec 350?

Answer 1

Pretty much any OBD II vehicle manufactured since 1995 will have one upstream oxygen sensor per cylinder bank, so your 5.7 will have two, fairly close to the exhaust manifolds. Then there is one before the catalytic convertor, and one after, these read the exhaust before and after the convertor to make sure it is doing its job properly. If you are thinking of repalcing and oxygen sensors for mileage or driveability issues, generally speaking it is the two right closest to the engine that have the largest effect on how the vehicle runs.