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Usual causes for this code:
Leaking PCV Valve Hose Contaminated MAF Sensor Leaking Intake Manifold Gaskets Bad O2 Sesor
This code is set becasue the PCM inputs indicate that the bank 2 (driver's side) cylinders are running too lean to be corrected. This is based on readings from the MAF, TPS, and Bank 2 O2 Sensor. I'd check the PCV valve hose first. Then I'd consider cleaning the MAF.
You should join the Expedition Owners Mailing List and pose the question to that group. See
Ed
On my 96 Mystique I DID have a leaky intake manifold (which I fixed) but the main culprit was the O2 sensor. After I replaced IT, the code finally went "bye bye". That's something like 2 years ago now.
P0171 is Fuel trim system lean bank *1* according to my book. P0174 is the same for bank *2*. If in reality both codes were set, the suspects are the fuel pump, fuel pressure regulator and fuel filter.
But since only ONE code is set, the chances are EXTREMELY high it is somethinf specific to one bank. My experience has shown this to be one situation where a bad O2 sensor is a high likelihood.
My point was that the info the OP posted was the code for bank 1 and the description for bank 2. Since the "local parts store clerk" read the code(s), there is some probability that there were actually both codes P0171 and P0174 stored and the clerk misread the output.
The OP could check at another place and have the code(s) read to see if there are really two codes set, since if it's both, the likely suspects change significantly. I just had P0171/P0174 on my truck and the problem was the fuel pump (Chev truck, but same OBD2 deal).
It could be any of the things you guys are talking about. However, since he was asking specifically about an Expedition, it is my guess it is either the PCV valve hose or the MAF since these are common problems for Expeditions. The PCV valve hose is know for cracking, allowing air in, and leaning out the mixture. This will set the lean codes. A dirty MAF will provide false air flow information to the PCM and cause it to think the parameters are out of range and set the codes as well. Of course the O2 sensor will too, but this is a not so common with Expeditions.
Ed
All depends on the code/s Both codes, agree 100%. Only one code? The likelihood heads very quickly towards the O2 sensor. OR an air leak that affects only the one side, which is EXTREMELY unlikely given the firing order/manifold design/etc. One half of the intake will generally affect 2 cyls on each bank.
Actually, there is a very good chance that only the one code might be set.... and what we do next (before any codes are cleared) becomes important.
Smart money would be to perform a KOEO On Demand test (I don't think the cheapy code reader that the parts store uses can do this.... ). This might reveal any pending codes.... say, if a P0174 was imminent.... These codes need to cross a threshold before they set to prevent nuisance codes.
If there are no pending codes, we can take a look at long term fuel trims (suddenly, we need an even better scan tool). If the LTFTs for the other bank are mimicking those of the first bank... we can be pretty sure that the problem is something that will affect both banks at the same time. If the LTFTs are close to zero for one bank but have crossed the threshold for the other, we would look for something that is affecting only one bank....
Mode 6 data might be one place to start (a better scan tool again) if it is a one bank only thing.... and here's that "threshold" thing again. Mode 6 data has a "misfire counter". A certain number of cylinder misfires are expected during normal driving... Mode 6 will count every last one of them on a per cylinder basis... A cylinder misfire will send a lot of oxygen down the pipe... and it is oxygen and only oxygen that is being measured... whether the misfire is caused by being lean, lean, lean - or because something is pig rich (there are other ways to decide this but I'm trying to keep this simple).
The important thing to remember... If a P017? is retrieved.... it does NOT mean that the other side is good to go.... it simply means that the other side may not have have crossed the threshold yet. And we need to find out if this is the case or not.
It isn't easy... but that doesn't mean it has to be a guessing game...
Which is why I suggested the OP have another place check the code(s), since he got the mixed report on the first read.
FWIW, the top model of the OBD scanners sold by Harbor Freight, the one with the text LCD display that can be had for a bout $60 on sale, while not a full blown scanner, will display the full freeze frame data which shows the fuel trims, speed, rpm, load, etc. A pretty decent unit for cheap, I keep it in my truck toolbox.
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