Car sounds awful and code P0171

[bluextc]

Any ideas? The exhaust has been sounding like crap for about a week now. I noticed one tailpipe seems to be sitting lower than the other, so I thought maybe it was an exhaust leak, but today the service engine light came on, so I hooked up the sct and got "P0171 - System too lean 1". I did a quick look to see if anything was obviously wrong, but everything seems to be still connected properly and no obvious cracks in the hoses.
I haven't installed anything lately, the last thing was the h pipe, but that was about 6 or 7 months ago and it's been running fine until now. Here's the list of mods that I think might apply, in case that's needed:
Mac cai
accufab 75mm tb and plenum, mac o/r h pipe, flowmaster 40's, sct xcal 2 with 93 tune.
Thanks for any help you can give me...

 

[4.6andSlicks]

Here's what my manual says:
*Check air cleaner element and housing for blockage
*Verify fuel level. ( I guess they think you ran out)
*Check for disconnected spark plug wires (maybe?)
*Check for vacuum leaks
*Check integrity of the PCV system
*Check to make sure the Mass Air Flow sensor is connected

If these are all good it moves on to some more intense stuff involving
BARO, LONGFT1, LONGFT2 and MAF V PIDs on fully warmed-up engine. This is beyond me.

The next step wants you to check fuel pressure. Have you done that yet?

 

[bluextc]

No, I'm not even sure how to... I guess I'm just going to get out there and check the things you listed for me today. I do have an appointment to take it to my shop a week from today already, I just hate to wait that long to fix the problem if I can figure out what it is before that...

 

[chuck943]

Anything that could cause to much unmetered air in engine or exhaust before it gets to O2 sensor. See PCM relies on measuring all the air coming into engine thru MAF then it measures the amount of oxygen present in exhaust and adjust the air/fuel mixture to a pre-calibrated level,if it can not get the oxygen content to read what it expects to see it will set the code, P0171 for bank 1 and P0174 for bank 2 for lean condition. Could be a vacuum leak,exhaust leak,not enough fuel,MAF sensor,etc. But since you only have code for one bank running lean I would concentrate on that side,like exhaust leak or leak around intake manifold on that bank,etc.

 

[bluextc]

Well, at this point I am pretty sure it's an exhaust leak... Car was supposed to go in next week, but I'm going to see if they can get to it sooner.
Thanks for the help.

 

[Dusstbuster]

Clean your Mass Airflow Sensor

My car was throwing random rich/lean codes for over a year, they were sporadic but annoying nonetheless. Put it on the dyno after doing what I think will be the end of my modding and found out my MAF was dirtier than a mofo and throwing the AF ratio off VERY bad. Make sure your MAF is clean as a whistle...