How To Test The MAF Sensor (2000-2004 2.0L Ford Focus)

This tutorial will take you through the process of testing the MAF sensor on the 2000-2004 2.0L Ford Focus using just a multimeter.

We'll start off by checking the sensor's signal, and if it's stuck, the next step will be to confirm the sensor is receiving power (VPWR) and that both Grounds (PWR GND and MAF RTN) are present.

By the time you finish with your checks, you'll know whether the MAF sensor is doing its job or if it's toast and needs to be replaced.

APPLIES TO: This tutorial applies to the following vehicles:

• 2.0L Ford Focus: 2000, 2001, 2002, 2003, 2004.

MAF TROUBLE CODE DIAGNOSTICS:

• P0102 MAF Signal Low Code Diagnostics (2000-2007 2.0L Ford Focus).
• P0103 MAF Signal High Code Diagnostics (2000-2007 2.0L Ford Focus).

Symptoms Of A Bad MAF Sensor

The mass air flow (MAF) sensor's job is to measure how much air is coming into the engine. The computer then uses that information to figure out how much fuel it needs to spray into the cylinders.

When the MAF sensor starts giving bad readings —or quits altogether— the computer can't fine tune the air/fuel mix correctly. The end result is poor engine performance, and you'll usually notice one or more of these symptoms:

• Check Engine Light: The CEL will be lit on the dash, and the computer will usually store one of these codes:
  • P0102: MAF Sensor Signal Low.
  • P0103: MAF Sensor Signal High.
• Lack of power: The engine may feel weak or hesitate when you hit the gas, especially under load or at higher speeds.
• Rough idle/stalling: The engine can idle unevenly or shut off on its own.
• Bad gas mileage: A bad MAF affects the PCM's ability to control the air-fuel mixture, making the engine run rich or lean, which kills gas mileage.
• Black exhaust smoke: If the PCM is adding too much fuel because of a bad MAF sensor signal, you may see black smoke coming out of the tailpipe.
• Hard starting: Sometimes the engine will crank but struggle to start.
• Hesitation or surging: You might feel the engine stumble or surge while accelerating or cruising.

If a P0102 or P0103 MAF sensor diagnostic trouble code (DTC) is stored in your PCM's memory, the following articles will help you understand what it means and how to use that information to fix the code and the MAF sensor problem:

• P0102 MAF Signal Low Code Diagnostics (2000-2007 2.0L Ford Focus).
• P0103 MAF Signal High Code Diagnostics (2000-2007 2.0L Ford Focus).

MAF Sensor Circuit Descriptions

In order to correctly diagnose the MAF sensor as good or bad, we need to know what each of the six wires coming out of the connector is doing.

The following table provides a brief description of each wire:

Terminal	Wire	Description
1	White with violet stripe (WHT/VIO)	IAT Sensor Signal
2	Green with red stripe (GRN/RED)	VPWR -12 Volts Input in RUN or START
3	Black with orange stripe (BLK/ORG)	PWR GND -Chassis Ground Input
4	Brown with blue stripe (BRN/BLU)	MAF RTN -MAF sensor Ground (provided by PCM)
5	White with blue stripe (WHT/BLU)	MAF -MAF signal
6	Brown with white stripe (BRN/WHT)	SIG RTN -IAT sensor Ground

Where To Buy The MAF Sensor And Save

The following MAF sensors are of known automotive brands (brands that I use myself):

NOTE: Not sure if the indicated MAF sensors fit your particular Ford Focus? Don't worry. Once you get to the site, they'll ask you for the specifics of your particular vehicle. If it doesn't fit, they'll find you the right one.

TEST 1: Testing The MAF Signal

The first step is to test the MAF sensor's signal and see if it responds when engine speed changes.

In normal operation, the voltage signal coming out of the MAF should climb as RPM goes up and drop as RPM comes down.

When the sensor goes bad, the voltage usually stays fixed at one value no matter how much the engine speed changes.

IMPORTANT: Leave the MAF plugged into its connector while doing this check. You'll need to back probe the connector or pierce the wire with a probe. This is the same tool I use and recommend: Goupchn 4mm Banana to Banana Plug Test Leads Kit (Amazon affiliate link).

Here's how to begin the test:

1. 1

   Set your multimeter to Volts DC.

2. 2

   Attach the black multimeter lead to the battery's negative (-) post.

3. 3

   Hook the red multimeter lead to the white with a blue stripe (WHT/BLU) wire using the right probing tool.
   
   This WHT/BLU wire connects to terminal number 5 on the MAF sensor connector (see diagram above).

4. 4

   Start the engine and let it idle.
   
   Once the idle evens out, take note of the voltage reading on your meter. It may bounce around a little —and that's normal. This initial number is your base MAF signal voltage.

5. 5

   Accelerate the engine by opening the throttle.
   
   You can do this by moving the throttle linkage at the throttle body or by having a helper step on the gas pedal.

6. 6

   The MAF signal voltage should climb as RPM goes up.

7. 7

   Release the throttle to bring the engine RPM back down.

8. 8

   The voltage should drop as the RPMs come down.

9. 9

   Repeat the rev-up/release test several times to confirm the results are consistent.

Let's see what your test result is telling you:

CASE 1: The signal voltage went up when you raised engine speed and dropped when you let off the throttle. This is exactly what should happen and it confirms the MAF sensor is working correctly.

If you've got a stored P0102 (MAF Signal Low) or P0103 (MAF Signal High), the issue behind those codes isn't happening right now —because the signal is responding to RPM changes instead of being stuck high or low.

CASE 2: The signal voltage stayed fixed at one number no matter how the engine speed changed. This usually points to a failed MAF sensor.

Before calling it bad for sure, there are a couple more checks to do. For the next test, head over to: TEST 2: Making Sure The MAF Sensor Is Getting Power (VPWR).

TEST 2: Making Sure The MAF Sensor Is Getting Power (VPWR)

For the MAF sensor to work and send an usable signal to the computer, it has to be supplied with 12 Volts.

In this step, we'll check that female terminal number 2 is actually providing between 10 and 12 Volts to the sensor.

The wire feeding terminal number 2 is the green with red stripe (GRN/RED) wire in the MAF sensor's 6-wire connector.

We'll do a quick voltage check with our multimeter (with the key in the RUN position, engine OFF) to confirm this 12 Volt power feed.

IMPORTANT: Be careful when probing the front of the MAF sensor connector. Don't force the multimeter test lead in, because if you damage the female terminal, you'll end up having to replace the whole connector.

These are the test steps:

1. 1

   Unplug the MAF sensor from its connector.

2. 2

   Set your multimeter to Volts DC.

3. 3

   Attach the black multimeter lead to the battery's negative (-) post.

4. 4

   Lightly probe the female terminal number 2 with the red multimeter lead.
   
   Terminal 2 should correspond to the green with red stripe (GRN/RED) wire of the MAF sensor's 6-wire connector.

5. 5

   Switch the key to RUN, but don't crank or start the engine.

6. 6

   The multimeter should show 10 to 12 Volts.

Let's interpret your test result:

CASE 1: The multimeter registered 10 to 12 Volts. This is the correct result.

The next step is to make sure the MAF sensor is getting chassis Ground, go to: TEST 3: Making Sure The MAF Sensor Is Getting Ground (PWR GND).

CASE 2: The multimeter DID NOT register 10 to 12 Volts. Double-check all your multimeter connections and run the test again.

If you still don't see 10 to 12 Volts, you can conclude the MAF sensor itself isn't the issue —since it can't work without a proper 12-Volt feed.

Your next step is to find out why this voltage is missing from the GRN/RED wire and resolve it, so you can get the MAF sensor working again and clear the diagnostic trouble code (DTC).

TEST 3: Making Sure The MAF Sensor Is Getting Ground (PWR GND)

Alright, up to this point you've:

• Verified that the MAF sensor's signal voltage stays fixed at one value no matter the engine speed (TEST 1).
• Verified that the MAF sensor is receiving 10 to 12 Volts (TEST 2).

In this test section, we're going to make sure that the black with orange stripe (BLK/ORG) wire, which connects to female terminal 3, is delivering chassis Ground to the MAF sensor.

The chassis Ground delivered by the BLK/ORG wire is available at all times regardless if the key is in the ON or OFF position.

IMPORTANT: Be careful not to damage the front of the connector or female terminal with the multimeter test lead, or you'll need to replace the connector.

These are the test steps:

1. 1

   Disconnect the MAF sensor from its electrical connector.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

   Connect the red multimeter test lead to the battery positive (+) terminal.

4. 4

   Gently probe female terminal number 3 with the black multimeter test lead.
   
   Terminal number 3 should connect to the BLK/ORG wire of the MAF sensor connector.

5. 5

   You should see 10 to 12 Volts on the multimeter.
   
   NOTE: Since this is a chassis Ground, it's available at all times whether the key is ON or OFF.

Now let's break down what your results mean:

CASE 1: The multimeter showed 10 to 12 Volts. That's the expected outcome and it confirms the MAF sensor does have chassis Ground.

The next and final step is to verify that your Ford Focus' PCM is providing sensor Ground (MAF RTN) to the MAF sensor. Let's go to: TEST 4: Making Sure The MAF Sensor Is Getting Ground (MAF RTN).

CASE 2: The multimeter DID NOT show 10 to 12 Volts. This means the MAF sensor isn't receiving chassis Ground.

Go back and double-check your multimeter connections, then repeat the test. If the reading still doesn't come up to 10 to 12 Volts, you can rule out the MAF sensor itself —since it can't function without the BLK/ORG wire supplying chassis Ground.

Your next move is to find out why this chassis Ground is missing and restore it.

TEST 4: Making Sure The MAF Sensor Is Getting Ground (MAF RTN)

In this last test section, we're now gonna make sure that the MAF sensor is getting Ground from the PCM. This MAF sensor Ground is known as the MAF RTN circuit in Ford speak.

The wire that delivers this Ground to the MAF sensor is the brown with blue stripe (BRN/BLU wire and it connects to female terminal number 4 of the MAF sensor connector.

IMPORTANT: Be careful not to damage the front of the connector or female terminal with the multimeter test lead, or you'll need to replace the connector.

CAUTION: This Ground is provided by the fuel injection computer internally. Be careful not to connect this wire to battery 12 Volts, or you'll fry the computer. The multimeter voltage test I'm suggesting in the instructions below is a safe way to check for the presence of Ground in this circuit.

These are the test steps:

1. 1

   Disconnect the MAF sensor from its electrical connector.

2. 2

   Place your multimeter in Volts DC mode.

3. 3

   Connect the red multimeter test lead to the battery positive (+) terminal.

4. 4

   Gently probe female terminal number 4 with the black multimeter test lead.
   
   Terminal number 4 should correspond to the BRN/BLU wire of the MAF sensor connector.

5. 5

   Turn the key to the RUN position but don't crank or start the engine.

6. 6

   You should see 10 to 12 Volts on the multimeter.

Let's see what your test results mean:

CASE 1: The multimeter showed 10 to 12 Volts. This is the expected result and it confirms that the PCM is supplying sensor Ground (MAF RTN) to the MAF sensor.

You can determine that the mass air flow (MAF) sensor is toast if you have:

• Verified the MAF sensor's signal stays stuck at one value no matter the engine speed (TEST 1).
• Verified the MAF sensor is getting 10 to 12 Volts (TEST 2).
• Verified the MAF sensor has chassis Ground (TEST 3).
• Verified the MAF sensor is receiving Ground from the PCM (this test section).

When you're ready to replace it, here's the MAF sensor from two known automotive brands I have used in the past and recommend without hesitation:

• Walker Products 245-3102 Mass Air Flow Sensor Assembly (Amazon affiliate link).
• Delphi AF10128 Air Flow Sensor (Amazon affiliate link).

CASE 2: The multimeter DID NOT show 10 to 12 Volts. This result means the MAF sensor isn't getting a sensor Ground (MAF RTN) from the PCM.

Double-check your multimeter connections and repeat the test. If the reading still doesn't come up to 10 to 12 Volts, you can rule out the MAF sensor as the problem since it cannot function without the MAF RTN Ground from the fuel injection computer.

Your next step is to figure out why this Ground is missing and restore it.

More 2.0L Ford Focus Diagnostic Tutorials

You can find a complete list of tutorials and wiring diagrams for the 2.0L Ford Focus in this index:

• 2.0L Ford Focus Index Of Articles.

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

• P0123 TPS Code Diagnostics (2000-2007 2.0L Ford Focus).
• How To Test The Throttle Position Sensor (2000-2007 2.0L DOHC Ford Focus).
• How To Test For A Blown Head Gasket (2000-2010 2.0L Ford Focus).
• How To Test Engine Compression (2000-2010 2.0L Ford Focus).