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Symptoms of a Bad MAF Sensor – Fix it like a pro!

In this post, we’ll learn all about your car’s MAF sensor, including all the top signs your MAF sensor is bad. I’m a mechanic, and MAF issues are top of the list when chasing engine performance issues. Very shortly, we’ll have this figured out. Let’s jump in!

The top 5 common symptoms associated with a bad MAF sensor include:

  1. No start with engine light on
  2. Starts then stalls
  3. Hesitation and poor acceleration
  4. Misfiring and hiccupping
  5. Rough idle

But there are a ton more symptoms, and I’ve covered them below.

In this post, you’ll also learn how to check your MAF sensor both with and without tools. We’ll cover how the MAF works, where you’ll find it, how to diagnose it, how to clean it, the difference between the two main types, and finally, how to replace it.

We’ll cover the three-wire, four-wire, five-wire, and six-wire MAF.

Maf-sensor

Index

MAF Failure Symptoms, Diagnosis & Fix

When a mechanic hears any of the following symptoms, they typically suspect the MAF may be at issue, but swapping parts out without first testing isn’t smart, especially as most of the following symptoms are not exclusive to a failing MAF sensor and some MAFs are on the spendy side.

Anyway! …Here are the top 13 MAF failure symptoms, what you can do to test them, and the likely fix.

It’s always worth cleaning a Hotwire MAF sensor before replacing it. Cleaning only takes a few moments, and I’ve covered it below.

  1. Engine hard to start – remove the MAF connector and attempt to start – if it starts, replace the MAF
  2. Engine starts then stalls – remove the MAF connector and attempt to start – if it starts, replace the MAF.
  3. MIL ON with any of the following codes present – P0100 – to P0104 – Test MAF
  4. MIL ON with lean codes P0171 or P0174 – Clean MAF sensor
  5. Rough idle – Clean the MAF sensor
  6. Rich fuel condition at idle (negative fuel trim numbers) and lean fuel condition on throttle (positive fuel trim numbers)
  7. Engine surging – Check air filter and clean MAF
  8. Engine misfiring – Check MAF wiring and clean MAF
  9. Poor or inconsistent throttle pedal response – Check MAF wiring and MAF output
  10. Strong smell of raw gas – Clean & check MAF operation
  11. Black smoke from the tailpipe – Check MAF operation
  12. Hiccuping on acceleration – Clean MAF and check operation
  13. Bad gas mileage – Clean MAF and check operation

Jump to more detailed hands-on testing with tools and without.

Common fault codes associated with a bad MAF sensor and what they mean:

  • P0100 – Mass or Volume Air Flow Circuit
  • P0101 – Mass or Volume Air Flow Circuit Range/Performance Problem
  • P0102 – Mass or Volume Air Flow Circuit Low Input
  • P0103 – Mass or Volume Air Flow Circuit High Input
  • P0104 – Mass or Volume Air Flow Circuit Intermittent
  • P0171 – System too Lean (Bank 1)
  • P0172 – System too Rich (Bank 1)
  • P0173 – Fuel Trim Malfunction (Bank 2)
  • P0174 – System too Lean (Bank 2)
  • P0175 – System too Rich (Bank 2)
  • P0300 – Random cylinder misfire

What is A MAF Sensor?

A MAF sensor is a small sensor located inside your car’s intake system just after the air filter. It’s a fully electronic sensor with no moving parts. They generally can’t be repaired, and apart from occasional cleaning (Hot wire type), they require no maintenance.

The MAF may look insignificant, but it’s critical for proper engine management. As clever as the PCM is, it can’t add the correct amount of gas without the MAF sensor.

Where is MAF Sensor Located?

The MAF sensor is located in the intake trunking after, or we say downstream of, the air filter. It is typically located in an easily accessible location at the top of the engine. 

Locate the air box, and you’ll find the sensor is close by. If you can’t find yours, you may not have one fitted. Remember, not all cars have one; some engines estimate airflow and instead rely on other sensors like the MAP, RPM, ambient temperature sensor, and throttle position to calculate the air volume inside the engine.

What Does a MAF Sensor Do?

The MAF is tasked with measuring the volume, density, and temperature of the air as it enters the intake system. (engine)

To appreciate just how important this little guy is, it might be helpful to know why the PCM needs to know how much air the engine has swallowed.

It’s all about the Air to Fuel Ratio (AFR). Gas won’t burn without air, and when I say air, I really mean oxygen. But too much or too little air in the mix won’t work either. The sweet spot for gas engines is 14.7 parts air to 1 part gas.

Move too far on either side of this recipe, and you’ll quickly notice some problems. Poor performance, Engine light on, Rough running, engine vibration, and poor gas mileage are all closely associated symptoms.

It’s important the PCM knows how much air is inside your engine. Only then can it run the calculations and add the correct amount of gas. 

In real-time, this all happens in fractions of a second, but the story doesn’t end there. The PCM relies on data from other important sensors, one of the most important is known as the oxygen sensor, also known as an O2 sensor. It’s positioned in the exhaust system, and its function is to measure oxygen levels in exhaust gases; this helps the PCM fine-tune the air-fuel ratio.

There are other less critical sensors involved too, but since this post is about the MAF sensor, we won’t delve into them here.

How Does A MAF Sensor Work?

MAF sensors use resistance, or changes in resistance, to measure the airflow and communicate with the PCM. The PCM is, as you know, the main computer or brain tasked with managing the whole process of running your engine, so it relies heavily on sensors to supply good readings in a timely fashion.

The broad concept of a Hotwire MAF sensor is simple: The sensor contains two sensing wires, one of which is heated.

As the sensing wires are placed in the intake air system, passing air naturally tends to cool the heated sensor wire, which in turn reduces its resistance and causes it to draw additional current.

The difference in current draw (temperature) between the two sensing wires is, therefore, proportional to the volume of air passing over the sensor.

A chip inside the sensor monitors the resistance (current draw) and converts the readings into a voltage or frequency output depending on the MAF type (Analog or digital. More about this later).

The PCM reads and understands that output is proportional to airflow. Armed with this information, the PCM can now calculate the correct amount of gas to add to the engine.

Many MAF sensors employ an integrated air temperature sensor which helps the PCM adjust calculations for ambient temperature changes.

It really is quite clever!

MAF Sensor Types

MAF’s come in 2 main flavors :

  1. The Hot Wire
  2. The Hot Film

Some older cars from the 80s used a mechanical Vane Airflow type, but they were prone to wear and were troublesome. You won’t find the vane type in any new cars today.

Although Hot Wire and Hot Film are similar and do the same job, they are obviously different, and the clue is in the name.

The Hot Wire uses a heated wire to help measure airflow, and the Hot Film type uses a more sophisticated Hot Film. 

How to tell if your MAF is a Hot Wire or Hot Film?

You can’t always tell which is which without popping off the air filter cover and peering down the ducting at the sensor.

A Hot Wire type will likely feature two parallel small wires. As a rough guide, you’ll find most domestic and Asian cars use the Hot wire type, and most European cars use the Hot Film type.

The Hot Film type is a more sophisticated type of sensor and conceals most of its workings inside a hard plastic shell. The Hot Film sensors employ an air passage to funnel and filter the air before passing it over a sensor, and most use twin sensors plus a temperature sensor.

The twin sensor helps the PCM get a more accurate reading and detects air direction, which fluctuates during the combustion cycle.

Both Hot Wire and Hot Film MAF sensors may be further categorized as either Analog or Digital, and this relates to how the sensors communicate airflow readings back to the PCM.

This is an important difference as it changes how we approach testing with a voltmeter.

We’ll cover how to tell the difference a little later, but first, let’s focus on the main differences between the Analog and the Digital sensor.

Analog MAF Sensor

As described earlier, a chip inside the MAF sensor monitors resistance and converts it to a voltage, which the PCM understands is proportional to airflow.

The analog MAF uses a sine wave voltage which may be read with a regular old voltmeter, and we’ll cover that a little later. 

Typically it will read 1.3 volts at idle and will move steadily and proportionally up to 4 plus volts at wide-open throttle (WOT). (Specs vary per make)

Digital MAF Sensor

The digital MAF is very similar, except for how it communicates with the PCM. The Digital MAF uses an on-off type signal (0 to 5 volts) known as a square wave (frequency) to signal airflow volume to the PCM.

A chip inside the digital MAF sensor monitors resistance and converts this resistance to a frequency that the PCM is programmed to understand. The frequency is proportional to airflow.

Typically 2000 Hz at idle, and the Hertz should increase smoothly and proportionately to about 10,000 Hz at wide-open throttle (WOT). (Specs vary per maker)

To measure a digital sensor, it’s best to use a meter capable of capturing frequency (Hz) or just read the grams per second (G/S) using a scan tool. (Covered below)

Typically 50 g/s idle and 150 g/s at wide-open throttle (WOT).

How to tell if your MAF is Analog or Digital?

Generally, you can’t tell by just looking at them. Either look up the spec of your model in a workshop manual or break out the voltmeter. Some makers will only use one type across the range; for example, all GM products use Digital MAF sensors.

However, running a test on the signal wire with the engine running will confirm which type of MAF you have.

At idle, an analog signal wire will offer about 1.3 volts and increase proportionally as your helper increases RPM to a WOT value of about four plus volts.

If you find a value of about 2.5 volts on the signal wire that increases erratically with RPM, then you have a digital MAF sensor fitted, and you’ll need a meter that reads frequency to test it.

Anyhow, the process of identifying and testing a digital and analog signal wire is covered below.

Testing a MAF Sensor without Tools

We can still make some useful checks on our MAF sensor without tools or a code reader. Your particular symptom will dictate where you start your diagnosis journey; typically, MAF issues come in two flavors.

  1. The car won’t start
  2. The car starts but has performance issues

If you suspect a MAF fault or want to eliminate it as a possible issue, go ahead and follow these simple no tools MAF tests and checks.

Run these tests if Your MAF problem fits into the category of “Car Won’t Start.”

  • Check the wiring connector is secure. 
  • Try wiggling the connector and tapping on the sensor while a helper attempts to start the motor.
  • Remove the connector, and try starting now – If the car starts, your MAF is indeed faulty.
  • Remove the air filter cover – Check if the filter is dirty.
  • Remove the filter cover to examine the MAF – Check if the hot wire is intact (Hot Wire type).

Run these tests if your MAF problem fits into the category of “Car Starts But has Performance Issues.”

  • Remove the air filter cover and check if the filter is dirty – Clean or replace the air filter.
  • Remove the air filter and check if the MAF sensor is dirty – If so, use a quality aerosol MAF sensor cleaner.
  • Check the wiring connector is secure – Tighten, clean, or replace corroded or loose pin terminals.
  • Try idling the engine and wiggling the connector – If the engine misfires suspect a damaged connector or failed sensor.
  • Try idling the engine and tapping on the sensor – If the engine stumbles or stalls, suspect a faulty MAF sensor.

These tests are good for on the fly testing, but it’s always best to run a test with a voltmeter or, better – a scan tool. And that’s what we’ll do next.

Testing a MAF Sensor With a Voltmeter

Generally, I wouldn’t test a MAF sensor in the shop with a voltmeter, the scan tool is way better, but testing with a voltmeter is obviously possible. We’ll cover testing with a scan tool below also.

There are, as you know, two subcategories of MAF sensors, analog and digital. Most tests are similar but testing each will differ when it comes to checking the output signal.

While we can identify the wiring of both the analog and digital MAF with a regular voltmeter, we won’t be able to check a digital MAF output; for that, we’ll need a meter with a frequency function or oscilloscope.

We’ll need to know which type of MAF we have when it comes time to test the output. And we’ll cover that next in sensor testing. 

Testing an Analog MAF Sensor

As said, most tests are the same for both the analog and digital, but to make sense of the readings, we’ll need to know which type we have.

Your MAF maybe 3, 4, 5, or 6 wire, and we’ll show you how to identify the pinouts as well as how to identify both the analog and digital MAFs in this section.

Identify Your MAF Wiring

For this test, we’ll need a basic voltmeter and some back probes or paper clips.

Testing 3 Wire MAF Sensor

Before making sense of test results, we’ll need to do two things:

Thing one – Identify the wiring pinouts (which wire is which circuit)

Thing two – Identify our MAF as Analog or Digital

Thing One – wiring identification process

Note: all cars will have their own wiring order and color reference. Some manufacturers like GM make life a little easier – GM color codes all their MAF signal wires yellow.

The ID process is as follows:

  • Set your meter to volts and ground the meter on the chassis
  • Turn ignition On (Pos 2), also known as KOEO (Key On Engine Off)
  • Remove the MAF block connector (Note MAF codes will be set)
  • Probe each of the three-block connector pins in turn (Harness side)

Typically you can expect to find the following:

  • 12 v – (Power)
  • 0 v – (Ground)
  • 5 v – (Signal wire)

If you are missing any of these, you may have a wiring issue; consult a wiring diagram.

Thing two – MAF identification process

With your wiring identified, plug the block connector back in.

  • Start the engine and allow it idle (if possible)
  • Back probe both the ground and the signal wire with paper clips (be sure they don’t contact each other)
  • Attach your volt meter, black probe to ground, and the red probe to signal wire
  • Have helper rev engine on your comand

Two outcomes are likely:

  1. You have a reading of about 1.3 volts at idle, which rises proportionally and smoothly with RPM – If that’s the case, you have an analog MAF fitted
  2. You have a reading of about 2.5 volts that jumps erratically with RPM – If that’s the case, you have a digital MAF fitted (see Digital sensor testing below)

Testing an Analog MAF

Now that you know what type of MAF we have, we can test it and understand the readings. You’ve already done most of the heavy lifting. Here are some of the checks you can make:

Remember in an analog MAF sensor – as air intake volume increases, voltage increases.

  • Observe the readings as a helper increases RPM – are they smooth, proportional, and without dropouts, 1.3 volts at idle is normal up to about 4 plus volts at Wide Open Throttle (WOT)
  • Tap on the MAF and check for the sudden drop in voltage or misfiring etc
  • Try wiggling the block connector and wiring loom and check for a sudden drop in voltage or misfiring etc

Repair wiring, clean, or replace MAF as needed.

4 Wire MAF Sensor Pin Outs

We won’t cover testing or identifying MAF type step by step, as the 4-wire process is the same as the 3-wire above. However, there is one change – we’ll leave the block connector plugged in for the wiring identification step, which makes ID a little easier.

With Key On Engine Off (KOEO), a typical four-wire MAF has the following pinouts (plugged in):

  • 12 v – (Power)
  • 0 v – (Ground)
  • 5 v – (Reference wire)
  • 1 v – (Signal)

5 Wire MAF Sensor Pin Outs

With Key On Engine Off (KOEO), a typical five-wire MAF has the following pinouts (Warm engine) (MAF plugged in):

  • 12 v – (Power)
  • 0 v – (Ground)
  • 5 v – (Reference wire)
  • 1 v – (Signal)
  • 3.5 v – (IAT sensor)

The IAT (Intake Air Temperature sensor) value varies with intake temperature. So this value will vary accordingly. To help confirm – when the connector is plugged out, the IAT pin voltage will jump to 5 v.

6 Wire MAF Sensor Pin Outs

We won’t cover testing or identifying MAF type step by step as the 6-wire process is the same as the 3-wire above. However, there is one change – we’ll leave the block connector plugged in for the wiring identification step, which makes ID a little easier. While this pin-out is typical, there are variations of it.

With Key On Engine Off (KOEO), a typical six-wire MAF has the following pinouts (warm engine)(MAF plugged in):

  • 12 v – (Power)
  • 0 v – (Ground)
  • 5 v – (Reference)
  • 1 v – (Signal)
  • 5 v – (IAT reference)
  • 3 v – (IAT signal)

The IAT (Intake Air Temperature) sensor value varies with intake temperature.

Testing a Digital MAF Sensor

If you ran your volt meter test and found that your values aren’t making sense, then you are in the right place; you’ve likely got a Digital MAF fitted.

We’ve already covered pin identification, so now we need to set up the meter. And as you already know, you’ll need a meter with frequency function and some back probes or paper clips.

Alternatively, just use a scan tool and read the grams per second G/S. (see testing with scan tool below)

In a digital MAF sensor, as air intake increases, frequency increases.

Digital MAF frequency test is as follows:

  • Back probe the signal terminal
  • Back probe the ground
  • Set meter to frequency (Hz)
  • Connect the red meter lead to the back probe signal terminal
  • Connect the black meter lead to ground back probe
  • Start the engine and check Hz at idle (about 2000 Hz)
  • Have a helper increase the RPM and observe a smooth increase in the Hz (about 10,000 Hz at wide-open throttle)
  • Tap on the MAF and check for sudden drop in frequency or misfiring etc
  • Try wiggling the block connector and wiring loom and check for sudden drop in frequency or misfiring etc

Repair wiring, clean, or replace MAF as needed.

Testing IAT Sensor

As you know, many MAF sensors include an IAT sensor. Below you’ll find some typical values for IAT resistance, but best to compare your test results against your vehicle’s spec. Remember, IAT is a negative temperature coefficient, meaning as temperature rises, resistance falls.

  • 0°C 32°F – 5500Ω
  • 10°C 50°F – 3600Ω
  • 20°C 68°F – 2400Ω
  • 30°C 86°F – 1660Ω

To test the IAT, you’ll need to know how hot the area around the sensor is. An infrared gun will give you an accurate reading.

Test as follows:

  • Ignition OFF
  • Remove the MAF connector
  • Using a voltmeter set to resistance (Ω)
  • Probe the IAT signal and ground pins (on the MAF, not the harness)
  • Compare against the spec of your vehicle

Testing MAF Sensor with a Scan Tool

A Scan Tool is the best place to start any diagnostic journey. It shortens the diagnosis process and helps prevent incorrect parts buying and fitting. Many inexpensive scan tools will allow for MAF sensor airflow observation. 

The process is as follows:

  • Locate the DTC port (Diagnostic Test Connection), usually located under the driver’s side dashboard by the driver’s footwell.
  • Turn car ignition ON (pos 2) and turn scan tool ON
  • Read and record any codes (take a pic with your smartphone)
  • Start and idle your engine (if possible)
  • Select MAF in the scan menu. Within this function, you’ll find the airflow measured in G/S (GPS (Grams Per Second)). 
  • Note the reading should be stable at idle (between 2 – 5 G/S depending on engine size) if not suspect a dirty or faulty MAF.
  • While observing the G/S, have a helper wiggle the MAF connector under the hood. A jump in the G/S reading or a stumble from the engine indicates a wiring or connector issue.
  • Have the helper tap on the sensor housing lightly with a screwdriver handle. The G/S and engine idle should remain stable; if not, the MAF is faulty.
  • Now steadily increase the RPM and observe the G/S value; it should increase steadily without dropouts. (150 plus G/S at WOT, but varies by engine size and make)

Watching this steadily increase can be tricky; switching the scan tool to a graph function is much easier to spot dropouts. Many inexpensive tools will include a graph function.

How To Clean a MAF Sensor

Cleaning the sensor is always worth a try if you suspect a MAF issue; it’s easy and fast. That said, successful cleaning is only possible with a good quality MAF sensor cleaner, and you should know only the hot wire type can be cleaned. Attempting to clean a Hot film isn’t going to help any.

Before cleaning, ensure the ignition is turned off and the engine is cold.

The cleaning process is as follows:

  • Remove the airbox – Most airboxes are tool-less removal
  • Remove the air filter
  • Locate the MAF inside the airbox
  • Prepare the MAF cleaner aerosol by shaking it up a little
  • Spray the cleaner directly and liberally over the sensor
  • Allow the sensor to dry for 10 minutes
  • Clean the airbox (use a damp cloth)
  • Clean the air filter or replace it

I just described the fastest way to clean them, but the more thorough is to remove the sensor. I use a small artist paintbrush to agitate the deposit on the hot wire while spraying with the cleaner to rinse away the crap. Be careful of the wire. They won’t take rough play.

Job done!

How to Swap Out a MAF Sensor

Swapping out a MAF is a simple plug-and-play deal. No special procedures are needed. That said, some MAF sensor fasteners are tamper-proof Torx which can be a pain unless you have the correct driver. But a small needle nose grips or pliers will MacGyver them off in jig time.

The process is as follows:

  • Ignition switch OFF
  • Disconnect the MAF wiring harness
  • Remove the MAF fasteners
  • Remove the MAF
  • Inspect the intake ducting for damage. Cracks or splits upstream of the MAF will cause lean codes. Damaged pipes or clamps must be replaced.

The fitting process is the reverse, however:

  • Avoid touching the sensor
  • Fit a new gasket if supplied; add a little lube to the gasket to aid fitting
  • If your sensor came complete with the housing, be sure to fit the sensor housing with the arrow pointing towards the engine.
  • Fit fasteners or clamps
  • Inspect the air filter and replace it if necessary
  • Fit pigtail connector. The wiring block connector must be in great shape. A damaged, loose or corroded connector or pins will mimic a failed MAF sensor.
  • Clear engine codes
  • Start and test drive
  • Scan for codes and call her good!

Job done!

How Long to Swap Out a MAF Sensor?

Replacing a MAF sensor is one of the fastest and easiest mechanics jobs. That said, there is a process; diagnosing process will likely take longer than the fitting process, and that’s because a good mechanic won’t jump to conclusions and throw parts at your car.

Doing so often results in unnecessary bills and pissed-off comeback (ex) customers.

They will instead read fault codes and verify the MAF has failed before fitting a new one. Then the tech will check and clear all codes from the vehicle, test drives the vehicle, and check for codes once more before calling it good.

You could expect a mechanic to spend thirty minutes diagnosing and as much time again fitting a new one. So, all in all, to diagnose and fit a MAF sensor – somewhere around an hour to 1.5 hours.

What Happens if You Don’t Replace MAF Sensor?

Ignoring a faulty MAF sensor will eventually cause the vehicle to stall or not start. Other things you’ll notice are poor performance, hard starting, and a stink of raw gas.

You may also notice black smoke from the tailpipe and a rough idle.

Can You Drive With a Bad MAF Sensor?

While it is possible to drive with a faulty MAF, doing so is a fool’s economy. Unplugging the MAF sensor will allow you to drive the vehicle (most), but it causes the computer to make estimates of fuel needed, which tends to be on the rich side.

Meaning your car will get poor gas mileage and isn’t good for your oil quality either.

You may find the following posts helpful:

Are OBD scanners universal?

How to use a fault code reader

OBD won’t connect to ECU