"BASICS" section. You said that the o2 sensor is detecting too little O2. on 2001 GMC Yukon

Shouldn't that be too much if it is a lean condition? And also, in the "HOw do I know if it's a MAF sensor problem?" section, you say to do the "truth test" with the engine idling. My scanner does not give baro, but kpa (atmospheris pr.) and with key on,engine off, I get a fairly accurate reading. With the engine running, I get a very different reading, as I would expect. What am I not understanding here, or is there a mistake in the article? Should the engine be idling for this test? Thanks.

Asked by for the 2001 GMC Yukon