Your friend was referring to O2 sensors that number more than one. Some cars have as many as 4, at least one of which is after the cat. converter. Your ECM is programmed for the number of sensors required. I have one o2 sensor in the header, and things work well. The sensor starts working in closed loop when the temperature goes above 600deg. F. As long as you can generate that kind of heat in the exhaust where the 02 sensor is, it will work perfectly. Also, as on my truck, if you have dual exhaust with no cross over pipe, one sensor in one of the headers will also work OK for the entire engine.
