Standard HEGOs aren't really oxygen sensors--they are more like oxygen switches. (Technically, this isn't really true either since they are actually voltage generators, but this isn't really important for this discussion.) Thus, they will have one voltage when they detect more oxygen than their switching threshold and another voltage when they detect less oxygen that their switching threshold (but it isn't usually quite a square wave function). Unfortunately, these two voltage values may vary depending on the HEGO part number (and how much contamination and aging has affected them). The RMS voltage values for rich versus lean (but still switching) will vary according to this variance. You would be better off to scope the signal (or do approximately the equivalent with a scan tool).
If you really want to do some tuning, you should replace your HEGOs with UEGOs. They cost the OEMs about 10 times as much, so they probably cost 2-3 times as much on the open market. :rolleyes: UEGOs have a continous voltage range where the voltage output is actually proportional (more or less) to the difference between oxygen concentration in the exhaust and in the atmosphere (yes, altitude affects your O2 readings).
