yeh above 4500 fuel system goes open loop and the lambda readings are purely for real time data and not fueling
plug a computer into your obd2 socket and watch how the lambdas behave
at idle once reached a certain temp they should be pulsing up and down every second between 0.1 & 0.8 volts and averaging at .45 volts
over a certain rev range they will be like a smooth line indicating how the car is fueling at certain revs, once the car drops back below a rev range it switches back into closed loop once certain conditions are met and should start pulsing up and down
the rears do not control fueling but it indicates how rich / lean the mixture is once passed through the cat and says hang on a sec we want to protect the cat here and not kill it, the graph shuold be a lot smoother and constant especially when the cat has reached operating temps
there has been a lot of discussion about fueling and the post cat lambda, well around 4500 and above it wont make any difference
i have noticed subtle differences using a sports cat with o2 simulator, with it unplugged and running normally
with sports cat the car runs lean at idle and with a normal cat it over fuels, very subtle differences in response and acceleration
symptoms of a faulty front mixture lambda will be like an electrical misfire when u rev around 1500 - 3000, u immediately think it is coil pack / spark related 🙂, also piss poor fuel consumption and stink of petrol
and on the 2.2 engine m8 if it is faulty i have seen them not throw up a fault code but instead indicate the rear is knackered which it probably is
i seen a 2.2 misfire like crazy at idle when revving and the front was near enough dead but it showed up the rear as faulty