Q: Check engine light is on with codes p0171 lean condition and p0303 cylinder 3 misfire

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Ive been working on this code for a month now but it keeps coming back. My car was blowing black smoke out of the exhaust, hesitating to accelerate, getting REALLY bad gas mileage, idling rough and just running poorly. I replaced the mass airflow sensor, upstream O2 sensor and those problems were resolved immediately. I reset the codes went through a drive cycle but when it was completed the light came on with code p0171 (system too lean) then later after that code another one showed up p0303 (cylinder 3 misfire). I sprayed the vacuum lines as best as I could with carb cleaner and no change in idle so im not guessing its a vacuum leak. I put a screwdriver up to the injectors and theyre all clicking consistently. I wouldnt guess the fuel filter is bad because of how much fuel it was using before. The fuel pressure regulator isnt leaking and seems to be functioning to the best of my knowledge. Im at a loss on what the problem could be.

My car has 170000 miles.
My car has an automatic transmission.

You did the right thing in checking for vacuum leaks, but I would take another good hard look at that. Not necessarily with the carb clean, but with the eyeballs. If there is a bad fit at the air mass meter, or a crack in the large hose there, that will give you a 171. Most of the P0171 codes I fix are due to air leaks. Another possibility is that the number 3 injector is not delivering. If the fuel isn’t going in on that one cylinder, it will cause a misfire and the O2 sensor will see it as a lean system. The only way to check that is to take the injector out and measure its output. If you can’t find the problem, contact Your Mechanic to have a technician come out to your home or office to check out your Mitsubishi and let you know what it will take to solve it.

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