Troubleshooting P0171 And P0174 (1996-2003 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer)

This article will help you to diagnose diagnostic trouble codes P0171: System Too Lean Bank 1 and/or P0174: System Too Lean Bank 2.

One of the biggest misconceptions, about these two diagnostic trouble codes, is that they accuse the oxygen sensors as being bad and this is just not true. That's right, these two codes do not identify the oxygen sensors as bad.

In this tutorial, I'll go into the basics of what causes these codes and I'll also offer you a testing strategy to successfully diagnose P0171 and P1074 diagnostic trouble codes.

ES You can find this tutorial in Spanish here: Cómo Probar: Códigos P0171 y P0174 (4.0L Ford) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 4.0L V6 Ford Aerostar: 1996, 1997.
• 4.0L V6 Ford Explorer: 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
• 4.0L V6 Mercury Mountaineer: 1998, 1999, 2000, 2001, 2002, 2003.

Important Suggestions And Tips

TIP 1: You'll need a scan tool (that has Live Data capability) to take advantage of my tips and suggestions in this article.

Now, having said that, I've geared this article towards the person that owns a generic scan tool with Live Data capability Actron CP9580 Scan Tool). You don't need to have a professional technician level scan tool (the scan tool in my photos is just a generic scan tool with Live Data).

TIP 2: Working around and engine, especially when it's running requires that you be alert and take all necessary safety precautions. Your safety is your responsibility, so think safety all of the time.

P0171 And P0174 Essentials

P0171: System Too Lean Bank 1.

• PCM's Adaptive Fuel Strategy has reached its rich calibrated limit for bank 1.
• In plain English, this means that the PCM is adding fuel like crazy to even out the air/fuel mixture because it has detected a vacuum leak in bank 1.
• The vacuum leak may be real or it may be a 'perceived vacuum leak' caused by some failed sensor.

P0174: System Too Lean Bank 2.

• PCM's Adaptive Fuel Strategy has reached its rich calibrated limit for bank 2.
• In plain English, this means that the PCM is adding fuel like crazy to even out the air/fuel mixture because it has detected a vacuum leak in bank 2.
• The vacuum leak may be real or it may be a 'perceived vacuum leak' caused by some failed sensor.

As I pointed out at the beginning of this tutorial, these two trouble codes don't point to a specific sensor and accuse it of having failed and this can sometimes cause a headache, if you don't have a troubleshooting strategy.

The thing that's gonna' help you nail down the problem (and thus the solution) is to remember that these two trouble codes are only describing a condition in which the computer is seeing an unmetered amount of air entering the engine, that no matter how much fuel it dumps (into the engine), it can not compensate for it.

Now, in case you're wondering what the heck is unmetered air:

• Unmetered air is any air sucked into the engine after the mass air flow sensor (remember, the MAF sensor's job is to meter [measure] the air so that the PCM can inject the right amount of fuel for that amount of metered air).
• For example: if the intake manifold gasket is bad and creating a vacuum leak, then the air being sucked into the engine at this point (which is after the MAF sensor) is considered unmetered air.

Usually this unmetered air, that the PCM thinks is entering the engine, is usually due to a very large vacuum leak but not always and this is what complicates things a bit. Several things can fool the PCM into thinking there is a vacuum leak, like a failing oxygen sensor or even a very dirty MAF sensor.

In this article, I'm gonna' explore several of the things and conditions that cause a lean condition. More importantly, I'll offer you a diagnostic strategy that will help you to troubleshoot the issue.

Where Are Bank 1 And Bank 2?

You may be wondering where bank 1 and bank 2 are located on the 4.0L Ford V6, well, using the illustration in the image above as a guide:

Bank 1 is the engine bank that holds the spark plugs for cylinders 1, 2, 3. This bank is the one on the passenger side of the engine.

Bank 2 is the one having the issue. Bank 2 is the engine bank that holds the spark plugs for cylinders 5, 6, 7. This bank is the one on the driver side of the engine.

The main sensors responsible for telling the PCM that there's a problem with the air fuel mixture are the pre-catalytic converter oxygen sensors. These two are also known as the upstream oxygen sensors.

What Does 'LEAN' Mean?

If you've been researching the P0171 and P0174 diagnostic trouble codes for any length of time, you've seen the term lean (and probably rich) along the way. In fact, this word is part of the trouble code definition.

Knowing what a lean and rich condition are will help you to understand what it is you're trying to solve. So here goes a brief explanation:

• The PCM is tasked with mixing the correct amount of fuel and air to create ideal combustion inside each engine cylinder. In tech terms, this mix is known as the air/fuel mixture.
• The PCM measures the amount of air entering the engine with the MAF sensor.
• Once it knows how much air is entering (and combining other sensor input like coolant temperature), it then injects the correct amount of fuel for that amount of air.
• When something like a major vacuum leak lets air into the engine (after the MAF sensor), the amount of fuel is less than the ideal for the amount of air coming into the engine thru' the MAF sensor and thru' the intake leak.
• When this happens, the air/fuel mixture becomes more air heavy and is then considered to be lean.
• Now, when the opposite happens and the air/fuel mixture becomes more fuel heavy for the amount of air entering the engine, the air/fuel mixture is considered to be rich.

TEST 1: Fuel Trim Check

The very first thing we'll do, to get to the root of the problem, is to see if the PCM is really seeing a chronic lean condition.

What makes checking this so easy, is that the PCM will let us know how lean or rich the air/fuel mixture is with its Long Term Fuel Trim values (for bank 1 and bank 2).

These Long Term Fuel Trim (LT FTRM) values can be viewed with your scan tool in Live Data mode Actron CP9580 Scan Tool).

This is what you need to do:

1. 1

   Connect your scan tool to the diagnostic connector and turn the key on.

2. 2

   Go to Live Data mode (on the scan tool) and scroll down to the PIDs labeled LT FTRM1 and LT FTRM2.

3. 3

   Start the engine and let it warm up to about 190°F (88°C).
   
   Letting the engine warm up to this temperature, before continuing to the next step, will ensure the accuracy of your test results.
   
   You can check the coolant temperature with your scan tool by scrolling to the PID labeled: Coolant.

4. 4

   Note the LT FTRM1 and LTFTRM2 values, once the engine has reached the indicated coolant temperature.

5. 5

   You should see values should be moving between a positive and a negative value. The range should be between 10% and -10% (negative 10%).
   
   If there is a lean condition present (either because a vacuum leak is present or due to something else), the LT FTRM values will be above 10% and will not come down to a negative value. They will usually hover around 20%.

Let's take a look at your test result:

CASE 1: The LT FTRM1 and LT FTRM2 values moved between 10% and -10%. This tells you that the lean condition is intermittent.

An intermittent lean condition can be hard to solve since the problem is not present all of the time. Take a look at the section: I've Done All Of The Tests And I Still Have Codes P0171, P0174 for some suggestions.

CASE 2: The LT FTRM1 and LT FTRM2 values were above 10% and/or near 20% and did not come down to a negative value. This result confirms that your vehicle is suffering thru' a bonafide lean condition and needs further testing. Go to: TEST 2: Inducing A Rich Condition.

TEST 2: Inducing A Rich Condition

OK, after confirming that you do have a bonafide lean condition, you can start doing some specific tests to find out where this lean condition exists, if it exists.

Now to find out if the lean condition is being caused by a real vacuum leak or by the oxygen sensor, the next step is to check the performance of the oxygen sensor or sensors. This can be very easily done and I'll go into some detail about it.

The best way to check the oxygen sensors (the pre-catalytic converter oxygen sensors) is to create a rich condition while observing your scan tool (in Live Data mode) to see if the oxygen sensor numbers go up to 800 milliVolts (0.800) to 1 Volt.

This is what you need to do:

1. 1

   With your scan tool still connected to the diagnostic connector and the engine running, scroll down to the oxygen sensor PIDs.
   
   These Parameter IDs (PIDs) will be labeled O2S11 and O2S21.

2. 2

   Make sure the coolant temperature, as recorded on your scan tool, is still above 190°F (88°C).

3. 3

   While you observe the oxygen sensor values, have a helper spray a little bit of carburetor cleaner into the intake manifold via a small vacuum hose with the engine running.
   
   I want to emphasize spraying carb cleaner spray into a small vacuum hose. You can not disconnect the intake air duct hose (while the engine is running) from the throttle body to spray carb cleaner into the throttle while the engine is running.

4. 4

   As soon as the carburetor spray hits the inside of the intake manifold (via the vacuum hose), your scan tool should read 0.900 Volts for both O2S11 and O2S21.
   
   You can repeat step 3 several times, if you need to make sure of your test result.

5. 5

   You can repeat step 4 several times, if you need to make sure of your test result.

Let's take a look at what your test results mean:

CASE 1: The O2S11 and O2S21 values shot up and stayed at 0.8 mV to 1 V. This is the correct test result and it lets you know that both oxygen sensors are OK and not the cause of the P0171, P0174 lean codes. The next step is to check for vacuum leaks.

CASE 2: The O2S11 and/or O2S21 values DID NOT shoot up as the carb spray was being sprayed. This test result tells you that O2 sensor is bad and needs to be replaced. This is the one causing the diagnostic trouble code.

As soon as you replace the bad O2 sensor, you can see if it solved the DTC by checking the Fuel Trims again. If the problem was solved, they will be oscillating between a positive and negative number constantly.

Other Important Tests

Now, if the above oxygen (O2) sensor test confirms that they are OK, now you can confidently conclude that the engine has a vacuum leak and that this vacuum leak is the one causing the P0171 and/or P0174 diagnostic trouble code(s).

Suggested TEST 1: With the engine cold, visually and physically check all vacuum hoses. The most common vacuum leak problem is the PCV tube's elbow, where it connects to the intake manifold.

Suggested TEST 2: With the engine completely cold, start her up and spray around the intake manifold's gaskets to check for leaks. If the carb spray hits a spot that is leaking, the engine RPMs will increase or decrease and this will pinpoint the source of the vacuum leak (before undertaking this test, see: Checking For Vacuum Leaks With Carb Spray).

Not only will the RPMs increase or decrease, but if the engine is warmed up enough, if you look at the O2S11 and O2S21 voltage numbers on your scan tool, they will max out a 0.8 to 1 Volts every time the leaking spot on the intake gasket sucks in the carburetor spray (remember, you can't let the engine get hot and continue to spray the intake manifold to cylinder head areas with carb spray).

If after visually and physically checking for vacuum leaks and none are found, the next step is to check fuel pressure.

Low Fuel Pressure Causing A Lean Condition

A fuel pump usually goes to the 'big gig in the sky' from one moment to the next and without any warning but sometimes, you'll have it die a very slow death. When this happens, it sends enough fuel to keep the engine running, but not enough for the PCM to effectively control the air/fuel mixture.

When this happens, the end result is a lean condition that will set the DTCs P0171 and P0174. In most cases, you'll also see the engine idle rough and/or misfire upon vehicle acceleration.

To check to see if the fuel pump in your Ford vehicle is not supplying enough pressure, you need to use a fuel pressure test gauge to check it.

Ford has made it pretty easy to check the fuel pump pressure, since they have installed a Schrader valve on the fuel rail how to connect a fuel pressure test gauge to. The following tutorial will help you test the fuel pump:

• How To Test The Fuel Pump (1993-2000 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
• How To Test The Fuel Pump (2001-2003 4.0L V6 Ford Explorer And Mercury Mountaineer).

Checking For Vacuum Leaks With Carb Spray

Spraying the intake manifold gasket with carb spray is one of the most effective ways to find a vacuum leak, but it does have its risks.

What risks am I talking about? It's the fact that the engine could backfire while you're spraying the intake carburetor cleaner spray and start a fire.

To minimize any possible risk of a back-fire and/or fire, this test should only be done when you have made sure the PCV tube's rubber elbow (at the intake manifold) is not torn or that it does not have dry-rot. Also, the engine should not be at its normal operating temperature (meaning that you should not perform this test on a hot engine).

Checking the intake manifold gaskets for vacuum leaks using this method may sound scary but I can tell you that this test is performed all the cross country, pretty much on daily basis, without problems or complications -when the above safety precautions are taken.

Most Common Causes Of P0171, P0174 DTCs

CAUSE 1: PCV tube's rubber elbow (where it connects to the intake manifold) is torn open and causing a major vacuum leak.

CAUSE 2: Intake manifold gaskets that are leaking vacuum.

CAUSE 3: Fuel pump that's going bad, but hasn't completely fried yet.

CAUSE 4: A dirty (contaminated) MAF sensor.

CAUSE 5: An exhaust leak right before the pre-catalytic converter oxygen sensors.

CAUSE 6: Bad pre-catalytic converter oxygen sensor.

I've Done All Of The Tests And I Still Have Codes P0171, P0174

So, you have done all of the tests, you have researched the codes to death (online and in repair manuals) and nothing you have done or replaced has solved the problem!

Trying to solve P0171 and P0174 DTCs can convert your Ford vehicle into a money pit. To avoid this you need to remember one very important thing:

Be Patient: Whatever part (whether it's an intake manifold gasket or an O2 sensor) is causing the codes, hasn't completely fried yet. This happens.

Parts don't always fail in an immediate fashion. So, the gaskets or vacuum hoses or the sensor could seal/work/function/ correctly most of the time, and every now and then they stop doing their jobs. This happens a lot! What can you do about it? Not much but wait till the problem is present all of the time.

Patience is a virtue. If you have checked for vacuum leaks and you have none and you have verified that the Fuel Trims are OK (which means the O2 sensors are OK too), then the best thing to do is to wait a few days and retest again. Whatever is failing will continue to fail until eventually the problem will be present to verify it with the tests described in this article.

The important thing to remember, is that you can use your scan tool to check the Fuel Trim values with your scan tool to verify the lean condition (that is setting the P0171 and/or P0174 codes) is actually present.

More 4.0L V6 Ford Explorer And Mercury Mountaineer Diagnostic Tutorials

I've written quite a few 'how to test' tutorials that may help you troubleshoot your 4.0L V6 Ford Explorer or Mercury Mountaineer. You can find the complete list in this index:

• 4.0L V6 Ford Explorer And Mercury Mountaineer Index of Articles.

Here's a sample of the tutorials you'll find:

• How To Test For A Blown Head Gasket (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
• FPDM Basic Operating Theory (2004-2010 4.0L V6 Ford Explorer And Mountaineer).
• How To Test Engine Compression (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
• How To Test The TPS (1997-2001 4.0L V6 SOHC Ford Explorer).