2001 F150 4.6V8 code 0171

I recently replaced all my coils as i has misfire issues and the obd diagnosed some faulty coils. i replaced them and disconnected the battery to clear the error code. Since doing that about 4 days later whilst driving up our local hill from 6300ft to 9000ft the warning light came on. i had the helpful guys as Kragen auto check the code and they came up with 0171 this time. They cleared the code but the light came on again the next day
What do i need to replace to fix this? is it in any way related to the coils? Could i have damaged something when changing them? couldit be that the engine management system is just resetting itself after i disconnected the battery? The engine is running well no missfire or lumpy running.

P0171 = System too lean bank 1

Check everything over very thouroughly and make sure that all the vacuum lines are hooked up correctly. Ford uses color coded plastic vacuum lines and likes to run them the whole lengh of the engine compartment. Remove any split tubing and inspect any vacuum lines for breaks or cracks. Especially around the coil packs.

Did this code just pop right after you replaced the coils? Or has it been going on for some time?

I suspect just a problem with a vacuum line.

I had knocked off the hose on the left (drivers side) valve cover, since putting this back on and replacing the PCV on the right side valve cover the code has not returned,so I am hoping this was the problem.
Thanks

Yep, you fixed your problem!