Simon and others have well covered questions you raise, Zenit - but some additional remarks…
It needs to be further stressed that the Lambda technology is based on detecting the presence or absence of oxygen molecules (O2) in the exhaust. Ie, lambda=1 means complete combustion; all fuel is burnt - no O2 or residual fuel present. Old technology, carbs and EFI, is unable, neither quick nor precise enough, to maintain this throughout all variations of load and throttle settings. So ‘our’ ECUs reads O2 from the O2 sensor - and continuously fattens the mixture if O2 is detected, and leans out if no O2 is detected.
And also; lack of fueling precision dictates that engines are set up to run fat, primarily to avoid the mixture going lean at any time when throttle settings are altered. And during throttle variations the Lambda circuit is ‘disabled’ to avoid this. So when using AFR its necessary to ensure that your mixtures setting conforms to the prescribed CO petrol spec settings - usually somewhere between 0,5 - 1% for EFI, and 2 - 3% for carbs, ref Simon’s comments.
Also, since Lambda relies on O2; there must be no air contaminating the AFR sensor - the exhaust tail pipe is not really the safest place. The best place for the sensor is one of the bungs on the exhaust downpipe, originally used for exhaust back pressure testing, but also used for the O2 sensor - which of course has the same ‘air free’ requirements as the AFR sensor. As the O2 sensor must(!) be disconnected while fuel mixture is adjusted - and obvious solution is to replace the O2 sensor with the AFR one. However, the 02 sensor must be heated to some 400C to work - and is placed accordingly…
xj6 85 Sov Europe (UK/NZ)