Hi my 98 F150SC 4.6 V8s check engine light just came on. I went to 2 different Auto zone stores.It shows code PO171. First guy tells me its the oxygen sensor and tries to sell me one. The second stores guy is older and gives me a print out the tells 4 different possible causes. 1.If bank one and two codes set together suspect fuel pressure or MAF sensor. 2. Oxygen sensor defective 3. Ignition misfire-repair.4. fuel injector problem. The truck runs fine so I dont think its 3 or 4. I am looking at obvious stuff now but I hate to play hit or miss with O2 sensors.I am going to clean the MAF( I learned how here) . We have E-check here and I have to get my plates this month so I cannot just ignore it. Thanks for any help.

 

I'm having similar problems, and I too need to get emissions tested soon. Try replacing the fuel filter as well if you haven't recently. Let me know what you find.

 

You only have the code for one bank, so it's not likely the MAF sensor. Maybe an exhaust leak on the bank 1 manifold...

It certainly could be a bad injector on that first bank. Since you only have DIS and not that COP ignition setup, pull the plugs on that bank and let us know what you find that sticks out on those plugs.

Bad o2 sensors would set their own code. Lean/rich with P0171/P0174 means the sensors are doing as they are designed to do. Not those codes say nothing about sensor malfunctions...

 

So if you are getting both error codes, where is the best place to start?

 

What is continiously missed is the code interpetation and what causes it.
A 171 says bank lean.
The PCM has fuel tables for each bank.
The code is telling what condition the table is shifted to in response to the problem that caused it.
A lean table means the motor is running 'rich' and has tried to lean it back and came to table 'limits' that sets the code.
You need to look for the causes of a rich condition.
It could be a leaking injector, an air leak, fuel pressure to high, regulator leaking/faulty etc. because the OX sensors sees the results of the fault and cause it to be reported as a table shift out of limits.
Reading the code meaning is where to many people go wrong including the stores that read the codes. If the person reading the code has no indepth knowledge of what causes it, he is in no position to advise on parts or procedure.
This is why techs go to school and /or study the control systems in order to be able to 'logic' follow the cause trail to the fault.

 

When I 1st got the 0171 and 0174 codes at the same time. I was told that It was a 02 sensor. I changed one at a time and I still got the same codes. At the end it turned out to be a vacume leak. A 10 dollar fix. Check the TB elbow 1st then the hose past the pvc. For the piece of mind just change them out. Good luck Ben

 

People always miss the real meaning of the codes. What the computer is saying is that the injector pulsewidth has reached the upper limit, for a particular volume of air... which means it has reached the limit of the ammount of fuel it is "supposed" to add for a given ammount of air, and the O2's are still reporting lean.

So the computer is saying to you.. Hey, I've added all the fuel I think is necessairy, and this piece is STILL running lean!!! So, I'm having to add MORE fuel than I am supposed to to maintain stoich.

It can happen as a result of using a high alcohol content fuel, or an intermittent missfire, or a vacuum leak, or a faulty O2 sensor, or an exhaust leak, a weak coil pack, bad spark plug/wires (or several bad plugs), or a stuck open or stuck closed injector, or low pressure on the injector rail. Or it could be mechanical, but that's not likely.

Explanation of my opinions:
Alcohol-- Fuels high in alcohol require more fuel per volume of air to maintain stoich. So, a high alcohol fuel would require that your computer add more fuel than normal for the particular volume of air, resulting in a lean code.
Missfires-- When you have a missfire, your engine doesn't burn the fuel for that cylinder. So logically this would give you a rich code right? WRONG. Your O2 sensors don't see fuel, they see air. And they see all the unburned air in the exhaust stream, so that would give you a LEAN code.
Vacuum leaks-- Your computer delivers fuel to the engine based on the quantity of air it's breathing in (based on the MAF sensor) It verifys (and fine tunes) this with the O2 sensors... When you have a vacuum leak,(meaning-air that is introduced AFTER the MAF sensor) the computers fuel calculations come out wrong. (the O2 sensors read lean) So in an effort to fix the problem, it adds more fuel. This may fix the symptom, and in the case of a lean code, it usually does. But the computer is only allotted a certian level of "adjustability" before it has to report a problem. And in your case, the computer has reached the upper limit of the "fuel allotment". Hence the lean code. This doesn't mean that your engine is actually running lean. In fact, it could be running perfectly stoich. What it means is, that the computer realizes that it is having to add too much fuel to maintain stoich, so something is wrong.

That's why the code is so vague. The computer is just reporting an observation. Not a part failure.

I would lean toward a vacuum leak FIRST. 2nd choice would be a missfire related to spark plugs/wires. 3rd choice would be an exhaust leak. 4th choice would be missfire related to coil.

The other reasons are very unlikely if you ask me, but they are possible. But the reason they are ulikely, is that you would have more codes if they were the cause...

I'm almost 95% on a vacuum leak. Check the pcv hose where it enters the intake manifold. Betcha it's rotten right there. If not, spray starting fluid around the intake manifold/vacuum hoses and see if the engine changes sounds... It will help you locate vacuum leaks BUT BE CAREFUL. If you have a shorted out spark plug wire and you spray starting fluid on it, it will burn.

How long have you had your spark plugs/wires? If they have close to 200,000 miles on them, then they might be the culprit. If not, then they are probably not the cause. Crank your engine tonight and pop the hood, Look carefully at all the spark plug wires and see if any of them are arcing to your engine block. Night time is an easy way to find a skinned or otherwise flawed spark plug wire.

Have you recently washed your engine? If so, then it could be a shorted coil pack, or shorted spark plug wires. Remove all of them and dry them off.

I hope this helps...

 

Great post Chester.

 

Thank you everyone for your help! Plugs and wires in Aug. Napas best wires and autolight plugs. I have just finished looking at hoses the pvc looked alright,but I trimmed the end to make sure it has got a good seal.went over TB air intake. We have had a lot of rain so I dried the coil packs with air and wiped them down.I went for a ride around 15 miles and no light yet. I have things to do tonight so I dont now if I can get out and drive it more tonight. I will let you know what happens Thanks again to everyone

 

Well i went around 18 miles and the check engine light came back on.I am going to loojk for any other vac leaks this Sat

 

Keep looking and inspecting, it's there somewhere.
The PCM won't respond right away after clearing the KAM until well into the drive cycle/s untll the PCM has had a chance to see the issue all over again and make the logic decision the tables are out of limits again.

 

Yeah, and the cool thing about code readers is that they erase the code, but they don't reset the KAM (keep alive memory). So if you try something new, and erase the code, the CEL will pop back up quickly if it didn't fix it. If you reset the code by removing the battery, you would have to do a lot more driving before the code will set again. That's because it resets the KAM and your truck has to go through all the bullcrap mentioned in Bluegrass's post before it can detect a problem.

..... These trucks are really cool!

 

So after looking at everything I could and not finding anything,Igot fed up and took my truck to a shop.I known the owner since high school. So they say that they get the same code plus one saying that the 2 bank is lean. They check for V leaks with some kind of smoke machine and tell they cant find anything.So they erase all the codes and tell me to drive it. He says it could possably be my MAF going bad or a intake manifold leak. The exhaust are tight. Is there any way to test a MAF?He told me to drive and it might go.It has to be malfunctioning for him to be sure. I need a Echeck for my plates by the end of the month. All this cost me $60.00.Thanks for any help.

 

It's not the MAF. It could very well be an intake manifold leak.

But , do this first - Replace the fuel filter . Test fuel pressure at the rail, Then test the regulator. Post back here after and let us know outcome.

 

This kind of situation seperates the men from the boys for diagnostics.

A good scanner can, in the hands of an operator that understands, burrow into the PCM program and look at the tables and various other items that would shift those tables out of range and set the codes. Example: looking at the MAF output signal for being within limits.
Then he should know where to look for the issue.
Dealer techs are trained to go to these depth but seldom take the time because of pressures to get the vehichle out the door for profits but it is there to be used when needed.
Like anything else, you have to pay the shop time for the investigation.
Just because there are codes and a little discription for them dosn't always make it an easy fix.
Like the difference between a good doctor and one that is to much in a hurry.
If a dealer also inspects the truck for emissions, they have an added incentive to go that extra mile to get there money out of the job.
Good luck.