i never unplug a battery for any mod or repair except airbag work, installing a new battery or sometimes for installing a starter.

if you are trying to find out if the cause of an existing lean code is fixed, i recommend clearing the code with the proper scan tool rather than unhooking the battery.

just my personal/professional experience.

 

That's what I thot I never have unhooked it either for doin mods or anything.

And yah I use my interceptor to check and clear codes. Im now leaning towards my maf being/goin bad. My idle is perfect bout 880 and boost is- 19 no idle surge that's bout 1/4 warm tho. Lean code is still there with the CEL also once I get my exhaust leak fixed ill clear it again see if it coms back. Im ordering a new MAF when I get my tax return back.

 

here's my input, I've heard and talked to a few people in the industry who cross leads after engine work (basically touch the disconnected red cable to ground, NOT cross the battery terminals) which probably does the same thing as simply disconnecting the battery all together, but I'm sure in places like the BCM and PCM there are small caps and integrated circuits which contain learned parameters from things such as deteriorating parts, O2 readings, engine friction etc which affect the operation somehow. minuscule effects here to say the least, and octane changes to say the most.
my .02

 

Back before OBDII cars, disconnecting the battery for X amount of time would dump the ECM memory back to a default program. When making any engine modification, this function would allow for the car to "relearn" and adjust it's fuel trims and spark advance based on the way the engine is now running. Newer cars can simply have the PCM/ECM fuse pulled, if equipped, and this dumps the memory as many of the OBDII formats "learn" how the driver drives and tailors the management program to that driving style. If you don't disconnect the battery or pull this fuse, it simply takes a bit for the PCM to realize the potential of the part added or altered. It won't hurt to leave it connected. There's really only a few things you want the battery disconnected for: 1. starter, 2. alternator, 3. fuel tank sending unit, anything related to the airbag system.