I got a 2003 f-150 with the 4.2 engine. There is 106,000 miles on it and I'm getting the bank one lean code. It's either going to be a leak or the 02 sensor itself. I checked all the hoses and they seem to all be in good shape. I read that sometimes the plenum going to the block can leak. I'm trying to figure out if it's the 02 sensor going bad or if it's some other problem. The code did pop up one time and I got it cleared. It lasted about a month clear before showing up solidly now. I do notice that about half the time I'm sitting at idle in drive, the idle will loop between about 700-850 but not everytime. Any suggestions? Also, if it is the o2 sensor, will a bosch sensor work the same as oem or cause problems? Thanks alot.

I would check the fuel pressure as a bad pressure regulator can lean out one side. I would use some carb. cleaner and check for a leak around the intake manifold. If you find a leak the engine will smooth out. Also spray around the fuel injectors checking for leaks.
Then follow ford guidelines for checking o2sensors.

Thanks for the info. Do you happen to have a link for ford's proceedure for checking the o2's?

http://www.popularmechanics.com/automotive/how_to/4219927.html
This is a link with some easy to read information.

Thanks again. I think I'm going to get a can and if I can't find a leak, I'll change the sensor. It does have over 100k on it. Hopefully it'll fix the problem.

Well, I got a few cans of carb cleaner and used a whole one on the manifold, hoses, vacuum lines with no change at all in idle. On top of it all, when I got the code pulled to double check, it was 171 and 174 (bank 1 and bank 2 running lean). So now what should I check? I'm thinking maybe the mass air flow but I'm not sure. Any more advice?

Look for any air leaks around the exhaust manifolds. Who ever is scanning the engine ask to watch the data stream it is much more informative than the codes.

It was just one of those cheap code readers the guys at autozone do. I don't know if it'll help, but I noticed when I'm at a light and I first stop, the idle jumps for a second then will calm down to normal. If I let the brakes up and then hit them(touching only brakes) again making the truck jump a little bit, a second later the idle will jump up again then go back to normal. It I just sit there doing nothing it is stable.

I would clean the idle air control solenoid. The codes say you are running to lean. Did you check fuel pressure. The sensors do go bad but I would want a data stream to see if they will go rich on a rev of the engine. You can change if you want to due to mileage. Ford has a procedure to follow but you need a better scanner.

Thanks for the info. I just cleaned the maf and am about to clean the idle air control valve. Afterwards, I'll reset the ecu. Hopefully the light will stay off; if not, looks like I'll have to check the fuel pressure.