Engine code light #P0171

oldfrmtrk

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Possible causes:

Oxygen sensor defective
Ignition misfire-repair
Fuel injector problems

Truck L 2001 145K on the clock.
Engine is running smooth, not misfiring.
Starts right up, power level not effected, and it’s not using an excessive amount of gas.

Any suggestion on where to start, should I stop running it until I get it fixed??

Any suggestions would be extremely appreciated..
 

operator1391

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i am thinking vacume leak......cheack ALL you hoses and also check the rubber 90 on the back of the lower intake....you will have to remove the supercharger to get back there
 

WhiTriCobra

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Should run a little erratic with a vac leak. Since your truck runs fine, could be a bad 02 starting to crap out. With that mileage and if they've never been replaced before, i would replace them.
 

oilwell1415

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I would leave the O2 sensors alone, they have their own codes when they go bad and they also have to be working properly to set your code and no others. You have either a dirty MAF or a vacuum leak.
 

FireGuy

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Should run a little erratic with a vac leak. Since your truck runs fine, could be a bad 02 starting to crap out. With that mileage and if they've never been replaced before, i would replace them.

Not necessarily, mine ran fine with a small vac leak, just a code with no other5 symptoms.

I would leave the O2 sensors alone, they have their own codes when they go bad and they also have to be working properly to set your code and no others. You have either a dirty MAF or a vacuum leak.

This is exactly right. MAF dirty, Vacuum leak (first choice unless you have done any recent intake work)
 

oldfrmtrk

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Thank you gentlemen!!!

I cleared the code this morning then drove the truck 50 miles to work, and the light never relit.

My gut feeling is the OX sensors, I have read that their life expectancy is 50,000 miles, so mine are on their third lifetime…

I appreciate and respect your feedback, may you all have an exhilarating fun filled day!

Thanks Art.
 

Dusten

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i had a an svt focus with an 02 shitting the bed, it would throw a light about once a month. never did replace it
 

WhiTriCobra

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With a small vac leak, sure it'll run fine but if it's out the back lower intake, you'll know. But with vac leaks, usually your rpms could be a tad high. I've had that problem on a few cars before. I just know i've had that same code, cleaned everything, check vac leaks and once i changed the 02's it went away. Why, cause it's the 02 most likely sensing the problem. It's electrical, so anything is possible. Can it be a vac leak, sure can. Dirty MAF, yes. The MAF would have been the first step and cleaned it. Vac leaks are not hard to detect, move stuff around, follow hoses and so on.
I've had a vehicle not start or it would stall, shift hard and so on...all because of a bad tps sensor.
 
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oilwell1415

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O2 sensors are not a maintenance item like they used to be. The only people that still recommend replacing them at 50k miles are the people that sell them and the ones that pass that on after hearing it from them.

O2 sensors do not cause this code. Changing the O2 sensors in itself will not turn a check engine light off. The vehicle has to go through three drive cycles without detecting he fault before it will go off. Ford does define a drive cycle, but in the real world there is no practical way to complete one without a dyno, you just have to drive until it happens. It could be 5 minutes or 5 weeks depending on your driving. Just because it happened to go off after you changed the O2s doesn't mean they fixed it. You did something prior to changing them that fixed the problem and it just took that long to do the drive cycles.

If you look up the service procedure for this code, the firs thing it will likely say is to check for codes that actually relate to the O2 sensors and fix those first. If there are no O2 codes it will send you looking for vacuum leaks, MAF problems, fuel starvation, et al and O2 sensors will never be mentioned again.

These codes get misdiagnosed more often than almost any other code I can think of by everyone from Autozone to the dealership. I can't tell you how many cars I've had pull into the shop after spending $1000 at the dealership on O2 sensors when all they needed was a $5 vacuum line. The misdiagnosis is perpetuated every time someone asks this question online because the first thing everyone says is throw O2 sensors at it instead of taking the proper steps in the first place.
 

boppracing

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We just finished a lightning with the same code.. Have a smoke test done, your most likley find a vacuum leak.. Usually at the EGR gasket and or the Iac gasket and throttle body shaft. If you have a scan tool its most likely showing bank 1 lean and eventually bank 2 lean.. Long term fuel trim will have reach 25% which will set the check engine light. Good luck with your project..
 

oldfrmtrk

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Over 200 miles on it, and no more light!

So: I guess I don't need to do a thing, well maybe burn some rubber right after I clear the work parking lot.....

Thank's youll.!!!!
 

242culmer

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Having the same problem P0171 & P0174 show up every time i start up my 00 SVT Lightning once i clear the code its gone until i have to start up again.... Going to remove the S/C the weekend to check the 90 behind there... i have checked everything else.... Maf etc...
 

242culmer

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Fixed the 90 that thing was torn to bits... now the ol girl feels great and doing well with MPG's fuel just went up to $5.75perGal so i may have to park her soon... :-(

Thanks
 

chuckdlightning

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I'm throwing the same codes on my 99 L (P0171 and 0174). I DEFINITELY have a vacuum leak somewhere. I can hear it. I'm having some trouble pin-pointing the leak, so I'll have to dig some. I'm experiencing no performance issues at all. If the previously mentioned 90-degree hose (under the s/c) was the problem; wouldn't it run poorly? When I get off work, I'll check around with a can of carb-cleaner to see if I can find it. Any pointers? I would assume that if the leak was on one of the EGR gaskets, I would also have an EGR code popping up.. Am I wrong? My boost gauge appears to be working properly; so can I emliminate any of those vacuum lines from my search? Any advise is much appreciated. Thanks
 

chuckdlightning

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I'm afraid that this is going to be the death of me! I found ONE vacuum line that had laid against the EGR tube and melted. I thought I had got lucky.. I happened to have access to the EXACT hose I needed off of a cobra motor in the shop at work. I replaced the hose, cleared my codes, then drove home. STILL have a freakin leak somewhere and my codes came back on within 25 miles of driving. I had time this weekend to go over all the lines; and i can't find anything.. awesome.....
 

The Professor

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O2 sensors are not a maintenance item like they used to be. The only people that still recommend replacing them at 50k miles are the people that sell them and the ones that pass that on after hearing it from them.

O2 sensors do not cause this code. Changing the O2 sensors in itself will not turn a check engine light off. The vehicle has to go through three drive cycles without detecting he fault before it will go off. Ford does define a drive cycle, but in the real world there is no practical way to complete one without a dyno, you just have to drive until it happens. It could be 5 minutes or 5 weeks depending on your driving. Just because it happened to go off after you changed the O2s doesn't mean they fixed it. You did something prior to changing them that fixed the problem and it just took that long to do the drive cycles.

If you look up the service procedure for this code, the firs thing it will likely say is to check for codes that actually relate to the O2 sensors and fix those first. If there are no O2 codes it will send you looking for vacuum leaks, MAF problems, fuel starvation, et al and O2 sensors will never be mentioned again.

These codes get misdiagnosed more often than almost any other code I can think of by everyone from Autozone to the dealership. I can't tell you how many cars I've had pull into the shop after spending $1000 at the dealership on O2 sensors when all they needed was a $5 vacuum line. The misdiagnosis is perpetuated every time someone asks this question online because the first thing everyone says is throw O2 sensors at it instead of taking the proper steps in the first place.
 

The Professor

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Thanks for the voice of reason!! I’m chasing lean codes. Can’t get my 04 through a smog test. I replaced the delta pfe and fixed the EGR codes only to join the too lean club. I used to be a Ford ASSET instructor so I’m a bit embarrassed to have my truck beat me. Hard to get Chemtool back to the PCV behind the supercharger. With less than 90 K on my truck I’m guessing it’s just old age. 17 year old truck!
 

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