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2007 Focus SE, 170,000 miles.

At 160,000 miles I replaced the valve cover and gasket, coils, spark plugs, pcv valve+hose, thermostat, removed the flaps in the intake manifold, and replaced the stock airbox with oem looking one that has a replaceable filter. Everything has been fine, getting 29-30mpg every tank.

The check engine light came on about a week ago, P0171. The car still runs completely fine, idles fine, still getting 29-30mpg. I replaced the air filter and fuel filter, that didn't help. I cant seem to find any vacuum leaks anywhere.

Where/how should I proceed? Cheapest or most likely solutions first?
 

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2007 Focus SE, 170,000 miles.

At 160,000 miles I replaced the valve cover and gasket, coils, spark plugs, pcv valve+hose, thermostat, removed the flaps in the intake manifold, and replaced the stock airbox with oem looking one that has a replaceable filter. Everything has been fine, getting 29-30mpg every tank.

The check engine light came on about a week ago, P0171. The car still runs completely fine, idles fine, still getting 29-30mpg. I replaced the air filter and fuel filter, that didn't help. I cant seem to find any vacuum leaks anywhere.

Where/how should I proceed? Cheapest or most likely solutions first?
2007 Focus SE, 170,000 miles.

At 160,000 miles I replaced the valve cover and gasket, coils, spark plugs, pcv valve+hose, thermostat, removed the flaps in the intake manifold, and replaced the stock airbox with oem looking one that has a replaceable filter. Everything has been fine, getting 29-30mpg every tank.

The check engine light came on about a week ago, P0171. The car still runs completely fine, idles fine, still getting 29-30mpg. I replaced the air filter and fuel filter, that didn't help. I cant seem to find any vacuum leaks anywhere.

Where/how should I proceed? Cheapest or most likely solutions first?
I’m in the same situation, and tbh I just cleared the code. I’ve done everything you have done also, have you came across a p0128?
 

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So the code referenced generally relates to computer sensing a lean air/fuel mixture, which can be traced back to a few possible issues. O2 Sensors may be old and not working correctly or even just be loose in their mounting slots...seen it myself, I have. If you haven't ever replaced them, or not in a couple years, this is a very likely candidate. Vacuum hose leaks stemming from items you removed during maintenance and did not put back quiet right to make a good seal, or forgot to reset the clamp in place...etc, or simply a bad line somewhere in evap system. One of the oft overlooked items is the MAF Sensor, just in front or behind the airbox depending on model year you have. This basically tells the computer how much air it has to work with for combustion, so if it's not reading right it throws a code that can easily be misinterpreted.

The MAF (Mass Airflow Sensor) is relatively easy to remove/clean; word of caution, do not try to clean it mounted on the car as you will fry your O2 sensors, which gets into $$. Remove it or the piece of intake mounted inside of and get a spray cleaner specifically for MAF sensors that will leave no residue. Let it dry thoroughly before reinstalling or you will fry your O2 sensors. Oh, and while doing this take off your battery ground terminal, which will reset your computer and clear the code, so you can start fresh and verify if there are any other issues lurking still. This also prompts the computer to 'relearn' and adjust so just be aware of the 'relearning curve' it will go through for a bit. Oh and be sure to reconnect the MAF sensor to the wiring harness; yup, seen that one too, lol.
 

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2007 Focus SE, 170,000 miles.

At 160,000 miles I replaced the valve cover and gasket, coils, spark plugs, pcv valve+hose, thermostat, removed the flaps in the intake manifold, and replaced the stock airbox with oem looking one that has a replaceable filter. Everything has been fine, getting 29-30mpg every tank.

The check engine light came on about a week ago, P0171. The car still runs completely fine, idles fine, still getting 29-30mpg. I replaced the air filter and fuel filter, that didn't help. I cant seem to find any vacuum leaks anywhere.

Where/how should I proceed? Cheapest or most likely solutions first?
Lol, it's always going to be the last possible thing you check, but you are never wrong to replace old filters on the car that need to be serviced anyway. Most often, and especially if you've been messing around with the intake, it will be your MAF Sensor throwing the code, and it just needs to be cleaned and reconnected with no air leaks. Secondly, O2 sensors control the fuel mixture based on a feedback loop with computer, so if one/both are bad it throws a code. DANGER: you will fry O2 sensors trying to clean the MAF sensor while it's still mounted in the car intake and/or remount it before it fully dries out...spraying stuff directly into the air intake/carbs is something you do on pre-emissions era cars, not one with O2 sensors.

If you already resolved your code issue, please post your outcome/fix as it may help someone else down the line.
 

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I definitely don't agree that it will fry your O2 sensor. Never have I ever seen that happen, and I have cleaned a ton of mass air flow sensors.

Examples of stuff sprayed into the intake cleaning the throttle body what do you use throttle body cleaner. Engines with direct injection we spray stuff into the intake manifold to clean the intake valves.

My personal opinion I believe it is a better option to buy a basic scan tool or even a adapter you can use with your phone so you can read live data, and actually diagnose the O2 sensor instead of throwing parts at it.




Sent from my LG-LS997 using Tapatalk
 

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Discussion Starter #6
I’m in the same situation, and tbh I just cleared the code. I’ve done everything you have done also, have you came across a p0128?
I ended up taking the MAF sensor out, cleaning it, and putting it back in. I haven't had another check engine light and it's been a couple months. Total cost was about $5 for the MAF sensor cleaner.
 

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I ended up taking the MAF sensor out, cleaning it, and putting it back in. I haven't had another check engine light and it's been a couple months. Total cost was about $5 for the MAF sensor cleaner.
I’m in the process of going to junk yards and looking for the housing to cut up and just do a “diy” cold air intake
 
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