Lexmex

The P0171 can clear itself, but it's better to just clear the code. I have seen a P0171 clear itself, because the code is still in the computer, but the CEL is out. Same thing with a P03XX code related to a misfire in an ignition coil.

artbuc

Thanks Lexmex. Over the last few days I have managed to find a couple credible sources who said they fixed P0171 by replacing the Bank 1 A/F sensor. In this particular case, I am helping a friend who lives 250 miles away and visits only a couple times per year. He is very reluctant to go back to his Lexus dealer because they sold him a complete brake job (including rotor resurfacing) at 25k and again at 50k. He doesn't know anything about cars and blindly did what his dealer recommended. He has about 80k on his RX and it runs beautifully. The MAF wires were not overly dirty but I had MAF issues on my old RX and my currrent Toyota Avalon
(V6 3.0L) with sensor wires not much dirtier than his. If the P0171 code persists, I believe I will change out his Bank 1 A/F sensor on his next visit. I couldn't find any disconnected vacuum hoses and his PCV valve/hose looked good.

On thing I find curious is that a dirty MAF often throws P0171 but not a corresponding code for Bank 2. Seems like a bad MAF would affect both banks??

Lexmex

It's more often the P0171 and not a bank 2 code. Reason being is that there is a shorter path on the back of the exhaust (i.e., piping that goes from the exhaust manifold to the main cat on the back of the engine is a shorter path than going from the exhaust manifold on the front (bank 2) back to the cat. You get any issue with say foul fuel, a bad ratio, etc., that's going to be the one that gets thrown first.

When I used to have a ram air system, you can imagine the kind of pressure that would get thrown in their at high speed and even with an open exhaust and a good high flow cat, things would get backed up there and trigger that bank 1, sensor 1 A/F sensor. I got new A/F sensors not too long ago and I rarely see any kind of code at all (P0171 is now a very rate sight).

ADKLEXUS

I had this code come up 2 hours after NYS inspection. Lucky the check engine light came on and code popped up after it passed. Strange coincidence. Going to clean the MAF Sensor and go from there. Running perfectly but fuel economy could be better.