Lean code due to voltage?

[Jeremy C]

OK, this is not for my 3S, but the non-3s forum doesn't get as much traffic and this is bugging the shit out of me...

My daily is an '03 Focus SVT. For what seems like forever now, I've been throwing a P0171 code (lean bank 1). The car always ran fine and got anywhere from 27-35mpg, so I didn't put a lot of effort into fixing it beyond replacing a couple parts and making sure no vac lines were cracked/torn/broken. If I ever reset the code or had the battery unplugged, the code would be back within 40 miles (usually much sooner).

After getting stranded with a dead battery a couple times, I decided it was time to replace the battery. The change in the car was immediate and extremely noticeable. I don't know how long the battery had been failing, but I don't remember the last time it cranked so quickly, and the car feels stronger in regular driving (which I'm attributing to an alternator that isn't trying to run all the electronics off a dead battery). The part that I can't figure out, though, is why my code hasn't come back. I've had about 200 miles of in-town driving and 300 miles of highway since replacing the battery, and my ever present CEL hasn't visited me.

Now, assuming I haven't just jinxed myself and go out to meet my old friend when I leave work, how the hell would low voltage cause a lean code?

The only theory I can come up with is that due to low voltage (I'm suspecting I was getting maybe ~9-10 volts constant) my injectors weren't getting the juice necessary to open completely. The ecu can only alter parameters so much and pressure is going to be constant, so once the ecu gets to it's max dwell without fixing the A/F to it's liking, it would set the P0171. Am I overthinking this or should I just give up and be happy it magically fixed itself?

[RealMcCoy]

P0171 is a fuel trim limit code... You need to look at your fuel trims and see if it's adding fuel.

The common issues with that car would PCV/crankcase vent leaks, and MAF problems.

But as a general rule, you can expect any modern computer system to go bat shit crazy if it doesn't get the proper voltage.

It also very well could have been affecting the fuel pressure.