I have been assuming, since the CEL only comes on right after I start the car, and never comes on while driving, that there only is a lean condition when I start the car. But then I read this from
The ecu internally stores two sets of codes, one readable with all working OBD II reader, one not so much, it then counts for a preset number of times/starts that the condition exists, if it does it'll throw the code and the light comes on, theres also the 'just happened' codes, and the happened on a previous run codes, pending vs stored. Usually if there is something amiss it registers the code internally within 10 seconds, monitoring the STFT to see if its >17% will give you an idea.
This implies that I might have been running the engine lean for hundreds of hours, which worries me.
And then there is this from
To answer one of your questions, yes the O2 sensor determines if the engine is running lean or rich. But the ECU is programmed to believe that a certain amount of air flowing through the MAF will require a certain amount of fuel. So if extra air is introduced (or lost) then the O2 can certainly compensate in real time, however the ECU knows that it is having to add more fuel then what is to be expected for the metered airflow. If my understanding is correct, a P0171, does not mean that car is actually running lean in the sense of what's actually being burned. What it means is that ECU is "seeing" through the O2 sensor that it has to add more fuel than is to be expected and so in corrected form the car runs ok, but if it were to run blindly based of calculations, it would be running lean. So it's like the ECU saying "Hey something is wrong with your system and it would be running lean, but I am able to compensate to get it back on track, but you should know something is whack". This of course is under closed loop, once you go to open loop (full throttle) and its all calculation based that's where the problem rears its head. When you least want to run lean.
And this implies that the P0171 code does not mean that the engine is actually running lean at all.
So. I am not at all sure what to think now