Hey, I'm about to order a Nordskog air/fuel ratiometer for my 9-3 because with the boost levels I'm seeing I want to keep an eye on the mix. I assume it just wires in in series with the first (before-cat) O2 sensor?
I've never owned a cat-equipped Saab so I'm probably not the ultimate source of wisdom on O2 sensors - but I do seem to remember that to get any real guide to fueling apart from seeing when you're running really rich or lean (something more akin to the 'idiot lights' for oil pressure etc. versus 'real' gauges) you need a wideband sensor rather than the OE mounted thingy.
The standard 02 or lambda sensor will only provide information to tell you if you are running lean or rich either side of a stoich condition or 14.7 - 1 afr.
It will not however tell you accurately how far either side of it you are running, for example you could be led into a false sense of security thinking you are running a rich afr at full boost only to be running 14 - 1 or similar.
This is where a wideband sensor is needed as it will give an acccurate picture not just around the stoich condition mentioned above.
Unfortunately there quite expensive
Ah yes of course, that was it - thanks Pete. The O2 sensor that came with your car is designed to work in a very narrow band around stoichometric (sp?) because it will have corrected the mixture to stoich long before it drifts off too far - and on full load where a richer mixture is called for, the O2 sensor measurement is ignored by the ECU anyways. So it all makes sense now...