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I have a question a have a 2006 ford focus with just over 100,000 miles all stock and it idles really rough the rpms on the gadge jumps up and down from a little above 1000 then it almost dies at idle when I pull up to a stop sign it stails but if I put it in neutral right before I come to a stop it won't die do you have any idea what could be wrong also the check engine light is on and the code that comes up is P0171.
 

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P0171 - System to Lean (Bank 1) The Adaptive Fuel Strategy continuously monitors fuel delivery hardware. The test fails when the adaptive fuel tables reach a rich calibrated limit. For lean and rich DTCs:
Fuel system
Excessive fuel pressure.
Leaking/contaminated fuel injectors.
Leaking fuel pressure regulator.
Low fuel pressure or running out of fuel.
Vapor recovery system.
Induction system:
Air leaks after the MAF.
Vacuum Leaks.
PCV system.
Improperly seated engine oil dipstick.
EGR system:
Leaking gasket.
Stuck EGR valve.
Leaking diaphragm or EVR.
Base Engine:
Oil overfill.
Cam timing.
Cylinder compression.
Exhaust leaks before or near the HO2Ss.
A SHRTFT-1,2 PID value between -25% to +35% and a LONGFT-1,2 PID value between -35% to +35% is acceptable. Reading beyond these values indicate a failure.

I'd check the easy stuff first- like leaks in the air intake, or a loose dipstick. If you've never changed your fuel filter- now might be a good time. Those should be changed every 25k miles or so. Most people only change it when the fuel pump goes out. We have procedures and How-To's on here about fuel pumps. It could also be a combination of several small things. A leak in the air intake is a big thing- at least in the way that it affects how the engine runs- but a bad fuel filter and a PCV leak could create the same effect.

Now what can you tell me about the car? How many miles? What repairs or maintenance have you done that affect the engine? What modifications have you done to the engine or exhaust? Do you have an SCT XCal or some other data logging tool?
 
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