I got this code from my check engine light, and after what I found in the internet there are a lot of possible cause it seems. Here is what I found.

The following is an overview of P0171 check engine light code possible causes.
Fuel System:
Leaking or faulty fuel pressure regulator
Plugged or dirty fuel filter or lines
Fuel pump weak or defective check valve
Injectors leaking or faulty
Low fuel pressure or running out of fuel
Leaking EVAP system components
Faulty FRP (Fuel Rail Pressure Sensor)

Air Intake System:
Vacuum leaks
Contaminated, damaged or faulty Mass Air Flow sensor
PCV valve leak or stuck open
Air induction turbulance caused by wrong filter
Oil dipstick not seated
Air leaks after the Mass Air Flow Sensor (MAF)
Oil coated aftermarket air filter

Exhaust System:
Any exhaust leak before or near the oxygen sensors

EGR System:
Vacuum line disconnected from EGR System Module (ESM)
EGR valve, tube or gasket leak
EGR vacuum regulator valve leaking

Secondary Air Injection System:
Mechanically stuck secondary air injection valve

What I do know what is not the cause are these:
-MAF been replaces just about 1 year ago by my Mazda dealer and they clean the air intake area as well. So is not that.
-PCV valve been replaced by me also about 1 year ago and I checked too...no leak there.
-Also just passed smog check a week before this check engine light turns on. So not the exhaust.
-Oil dip stick is fine.
-Also just used one of those Lucas fuel injection cleaner for the tank of gas before the engine light turn on. Most likely not the fuel injection system.
-I know my 99 protege has no fuel filter according to mazda dealer, so can it be the fuel line?

All that is left for me almost sounds like only the EGR system left to check. The thing is, my car still drive like normal, nothing seems to be wrong, and gas mileage also normal, in fact is slightly better than what I normally get after I used the Lucas fuel injection cleaner. Only thing I notice in my engine compartment is low in radiator coolant level in the reserve tank not in the radiator or air filter is a little dirty. But air filter been dirty before in the pass and never trigger check engine light. Also, I wouldn't think radiator coolant level has to do with this code. So any idea or insight of what it might be would be very much appreciated. I am on the verge of just bringing it in to a mechanic to diagnose what really is the cause. Been almost a week since I got this check engine light but did not feel anything unusual with the car that I drive everyday. Thanks in advance.

 

P0171 on 2001 protege W/2.0 L ended up being a rip in the seam of the intake hose. Drove me bonkers trying to find the prob. When the engine idles, everything fine. When accelerating, the engine torques and the unseen rip in the intake hose opens, sending way more air to engine than what the MAF told the ECM . long story,but easy fix
(if thats problem).......replace in air metering hose from air box to intake manifold

 

My 02 same problem I found the cracks in the ribs of the air tube by running my fingers around the air tube. I have sealed the tube until I find one. The R&R is very easy.

 

I still have the problem ... the MAF sensor , O2 sensor is new and I don`t know what the problem is ....

 

Just went through this a few weeks ago found a small leak by bank 1 injectors at the intake . I was looking for a vacuum leak as a few videos on YouTube will point you to do. After checking all hoses for cracks or leaks i then started to spray leak detect on various spots I found a injector seal that was bubbling a bit so I ordered a o ring set and replaced them all . It was all good but when i sprayed leak detect around the injectors by valve cover I could here a sucking noise and engine started to shake. I took off fuel injector rail again. After close inspection I could see a small gap between gasket and intake housing. I took a chance and sealed it with gasket/maker sealer and it worked .I know its not the correct fix but saved me alotta work. If it goes again then ill order new gaskets and replace them. But hell the car is 22yrs old so if the fix lasts so be it...Good luck