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Originally Posted by BAN SUVS
The tail-pipe style widebands used by tuners (at least, the ones I've known personally, Shiv at vishnu and Nate at S-Squared) are heated, just like OEM narrowband units. They are also calibrated for use in that spot- they'd read improperly if used in the downpipe. We could probably debate just how precise the readings from either method is, but since they read consistently, and so few professional tuners have issues with overly rich or lean tunes, it's probably safe to use that method.
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Tail pipe wide band calibration is a touch and go thing at best. To do it reliably, you have to do it for each and every car, and all of the cats have to be at full functional temperature, and then in a range that the stock O2 before the cats is accurate, you test and compare, probably at a couple different load/flow rates.
I doubt there is a single accurate calibration number even for two "identical" cats for how they affect O2 readings. And before the CATs is what you care about the ratio because the cats job is to burn up left over hydrocarbonc which is the A/F sensor is reading.