What Does The MAF Sensor Do? (4.0L Ford Explorer)

The mass air flow (MAF) sensor is the go-to input sensor your 4.0L Ford Explorer or Mercury Mountaineer's fuel injection computer uses to calculate the amount of air entering the engine. This functionality makes the MAF sensor a critical engine management system component.

So when the MAF senor fails and stops reporting to the fuel injection computer, engine performance will suffer.

This article will answer some of the most commonly asked questions about the MAF sensor on the 4.0L Ford Explorer and Mercury Mountaineer.

APPLIES TO: This tutorial applies to the following vehicles:

• 4.0L V6 Ford Explorer: 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
• 4.0L V6 Mercury Mountaineer: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.

What Does A MAF Sensor Do?

The mass air flow (MAF) sensor's job is to measure the amount of air passing through its assembly. It then reports this measurement to your 4.0L Ford Explorer or Mercury Mountaineer's fuel injection computer.

To be more specific, the mass airflow sensor's output signal increases as the engine RPM increase.

As engine RPM decrease and less air passes through the sensor's assembly, its output signal decreases.

With this air intake information (the MAF sensor reports), the fuel injection computer can now calculate the correct amount of fuel to inject into the cylinders and fine-tune other engine management operations that ensure the engine runs optimally and efficiently.

What Happens When A MAF Sensor Goes Bad?

When the mass air flow sensor fails, it stops measuring and reporting the amount of air entering the engine to the fuel injection computer.

Without this information, the fuel injection computer lights up to check engine light with a mass air flow sensor trouble code. As a result, and as mentioned earlier, engine performance suffers.

In the next section, I'll go into some of the specific symptoms you'll see when the MAF sensor fails.

What Problems Can A MAF Sensor Cause?

A bad MAF sensor will cause one or more of the following symptoms:

• Bad gas mileage.
• Lack of power.
• Black smoke coming out of a tailpipe.
• Check engine light is illuminated by one or more MAF sensor trouble codes.
• Rough idle.
• The engine may start and immediately stall.
• Extended engine cranking (the engine has to be cranked a long time for it to start).
• The engine may not start.
• On OBD I vehicles, the check engine illuminated by one of the following trouble codes:
  • 157: MAF Sensor Circuit Low Input
  • 158: MAF Sensor Circuit High Input
  • 159: MAF Sensor Circuit Voltage Higher Or Lower Than Expected
• On OBD II vehicles, the check engine illuminated by one of the following trouble codes:
  • P0102: MAF Sensor Circuit Low Input
  • P0103: MAF Sensor Circuit High Input
  • P1100: MAF Sensor Output Voltage
  • P1101: MAF Sensor Output Voltage

How Can You Tell If A MAF Sensor Is Bad?

Most folks are under the impression that the MAF sensor diagnostic trouble code confirms that the MAF sensor has failed. But, unfortunately, this isn't the case.

The MAF sensor trouble code only alerts that there's an issue with the fuel injection computer's input from the MAF sensor and nothing more.

The only way you can determine the MAF sensor has failed is by testing its output signal, which you can do in several ways.

See the next section to find out more.

How Can I Find Out If The MAF Sensor Is Bad?

You can easily and quickly determine if the MAF sensor has failed by performing a multimeter test. This test is one of the many ways to determine if it has truly failed.

Whether you're using a multimeter, an oscilloscope, or a scan tool to test the MAF sensor, your goal is to check the MAF sensor's output signal.

Is the MAF signal stuck in one value as you increase/decrease engine RPM? Or is it correctly reacting to the increase/decrease of engine RPM? These are the questions your MAF sensor test should answer.

Your MAF sensor diagnostic test should include the following steps:

• Check for MAF sensor trouble codes with a code reader or scan tool.
• Confirm that the MAF sensor is receiving 12 Volts.
• Confirm that the MAF sensor is receiving Ground.
• Confirm that the sensor's MAF voltage signal increases/decreases as the engine is accelerated/decelerated.

If the MAF sensor's output signal does not increase/decrease as you accelerate/decelerate the engine, then you can conclude that the MAF sensor is bad and needs replacement.

Testing the mass airflow sensor is something you can do without taking your 4.0L Ford Explorer (Mercury Mountaineer) to an automotive repair shop (but you can).

I've written a tutorial on how to test it with a multimeter. This tutorial doesn't cover all 4.0L Ford Explorer and Mercury Mountaineer model years, but it will give you an idea of what the test involves in those particular cases. You can find the tutorial here: How To Test The Ford Mass Air Flow (MAF) Sensor (at: easyautodiagnostics.com)

Can I Drive My Explorer (Mountaineer) With A Bad MAF Sensor?

When the MAF sensor fails, the fuel injection computer activates its 'limp mode' (also known as 'limp home mode' or 'limp-in mode').

The 'limp mode' limits engine performance to a considerable degree to ensure the fault doesn't cause any severe damage to the engine.

As you can already surmise, 'limp mode' will not provide a pleasant or safe driving experience.

Also, depending on the severity of the driveability issues the faulty MAF sensor is causing, you won't be able to drive the vehicle at all.

More Ford 4.0L Explorer (Mercury Mountaineer) Tutorials

You can find a complete list of 4.0L Ford Explorer (Mercury Mountaineer) tutorials in this index:

• Ford 4.0L Index Of Articles.

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

• How To Test The Fuel Pump (Ford 4.0L Aerostar, Ranger, Explorer).
• How To Test Engine Compression (Ford 4.0L).
• How To Troubleshoot A No Start (Ford 4.0L).
• How To Test For A Blown Head Gasket (Ford 4.0L).