Many cars use a mass air flow (MAF) sensor to measure the amount of air entering the engine. When the device goes bad, it can lead to various symptoms, ranging from an illuminated check engine light to rough running and misfiring.
As you might guess, a bad MAF sensor can be a huge inconvenience. What’s more, the problem can damage other parts of your vehicle, such as the catalytic converter, if left unchecked.
So, if you think your MAF might be faulty, it’s a good idea to address the issue as soon as possible.
Mass Air Flow Sensor Symptoms
Because the MAF is a primary input to your car’s computer for fuel control (more on that later), the sensor can cause several noticeable symptoms when it fails. Some of the most common air flow sensor problems include:
Illuminated Check Engine Light
Often, when there’s an issue with the MAF sensor, your car’s primary computer—often referred to as the powertrain control module (PCM)—will turn on the check engine light and store a diagnostic trouble code (DTC) in its memory.
Rough Running and Misfiring
Without an accurate signal from the MAF sensor, the PCM won’t know how much fuel to give the engine. That can either cause the engine to run rich (too much fuel) or lean (too little fuel). As a result, the combustion process—the ignition of the air/fuel mixture inside the engine—will be incomplete.
The outcome is an engine that runs rough and misfires.
Lack of Acceleration, Hesitation, and Stalling
A bad MAF sensor can trick the PCM into starving the engine for fuel. When that happens, the vehicle may exhibit a lack of acceleration. Similarly, a faulty MAF may cause your car to hesitate and stall.
Reduced Fuel Economy
A faulty MAF sensor can throw off your engine’s performance, leading to an increase in fuel consumption.
What Does a Mass Air Flow Sensor Do?
As was mentioned, the MAF sensor measures the amount of air entering the engine. The PCM uses the MAF sensor signal when calculating how much fuel to give the engine—more air equals more fuel.
There are several different types of MAF sensors found in different vehicles. The two most common designs are hot wire and hot film. Both types of sensors operate in a similar manner.
To measure airflow, the PCM continuously provides electrical current to heat the hot wire or hot film element. Airflow cools the element, reducing its electrical resistance, and creating an increase in current demand. The PCM uses the increase in current demand to determine the amount of air entering the engine.
It’s important to note that not all cars have a MAF sensor. Some models use what’s called a Speed Density System, instead. With this setup, the PCM uses input from the manifold absolute pressure (MAP) sensor and various other sensors when calculating fuel delivery.
How to Diagnose a Bad Mass Air Flow Sensor
A wide range of problems can mimic a bad MAF sensor, so it’s a good idea to test a suspect sensor before you rush out and buy a new one.
Before getting started, however, you’ll want to have the vehicle’s repair information handy. Manuals, such as those from Chilton, are good, but a subscription to a repair database (e.g, ALLDATA or Mitchel 1 DIY) is even better. You can find more information on accessing quality repair information in our article on repair manuals.
Note: The following are general guidelines for educational and entertainment purposes only. Consult your vehicle’s factory information for specific repair instructions and recommended safety procedures.
Step 1. Check for Diagnostic Trouble Codes (DTCs)
If the check engine light is on, you’ll want to retrieve the corresponding diagnostic trouble codes using a scan tool or code reader. These days, you can even get an inexpensive code reader for your smartphone.
Is there a MAF-related DTC stored in the PCM’s memory? Then there may be a problem with the MAF sensor or its circuit. As was mentioned, air/fuel ratio and misfire codes may also point to a problem with the MAF sensor (though, there are many other possibilities, as well).
Keep in mind: Sometimes, a bad MAF sensor won’t set a code, especially on older vehicles with a type of self-diagnostics called OBD-1.
Step 2. Perform a Visual Inspection
Next, you can move on to performing a visual inspection. You’ll want to look for issues, such as damaged wires and poor connections. Make sure that the MAF sensor’s electrical connector is clean and tight.
Repair any issues found during the visual inspection, clear the DTCs, and see whether the problem returns.
Step 3. Test the MAF Sensor
Testing a MAF sensor can be tricky. Hot wire sensors output an analog voltage signal, while hot film sensors create a square wave digital frequency pattern.
It’s important to know what type of sensor you have—and you should always consult the factory repair information before testing.
Check the MAF Sensor Data with a Scan Tool
You can check the operation of both hot wire and hot film MAF sensors with a scan tool. A fancy scanner isn’t necessary for this step—you can use a basic tool, as long as it displays OBD live data.
The data parameter you want to view is usually labeled airflow rate and is measured in grams per second (g/s).
Here’s how it’s done:
- Find the airflow data parameter ID (PID) on your scan tool.
- Turn the engine on, bring it up to operating temperature, and leave the vehicle in park. Let the engine idle while you monitor the PID.
- At idle, the approximate air flow rate (in g/s) should loosely mirror the size of the engine (measured in liters). For example, if you have a 1.8L engine, the reading should be close to 1.8 g/s.
As you’ll see in the image below, which is of a Honda 1.8L engine with a healthy MAF sensor, there’s a reading of 2.27 g/s at idle.
Note: The idle airflow/engine displacement correlation is an imprecise, rule-of-thumb measurement that can vary by vehicle. Some automakers include the MAF sensor specifications in the factory repair information. It’s a good idea to consult a repair manual or repair database for this information.
- For this test to be useful, you should also monitor the air flow PID at wide open throttle (WOT).
- Have an assistant drive while you monitor the air flow PID on the scan tool (or use the tool’s recording capability if equipped). It’s best to view the air flow PID as a graph if your scan tool has that capability.
- Have your assistant operate the vehicle at WOT.
Note: The transmission may need to be put into a lower gear to create a sufficient load on the engine.
- At WOT, the MAF sensor reading should be approximately 40 times the engine displacement. For example, if your vehicle has a 1.8L engine, the MAF reading should be approximately 72 g/s (1.8 x 40 = 72).
- If the engine has variable valve timing (VVT), the MAF sensor reading should be approximately 50 times the engine displacement. So, if our 1.8L engine has VVT, the reading should be closer to 90 g/s (1.8 x 50 = 90).
- Should you find the readings to be far below these values, chances are, the MAF is either dirty or faulty.
- You can test the MAF further using the following approaches.
Checking the Output of a Digital Frequency (Hot Film) MAF Sensor
Hot film MAF sensors output a digital waveform pattern that looks like little square humps. The frequency of the humps increases as air flow through the MAF increases.
To check the output of a hot film style MAF sensor, you’ll need either a digital multimeter with a frequency (Hz) setting, an OEM-level (i.e., expensive) scan tool, or an oscilloscope. The scope is the best choice since it allows you to see minor glitches in the MAF output signal.
Our sample vehicle for this test is a 1998 Chevy Astro with a 4.3L engine and a known good MAF sensor. Chevy’s specifications for the MAF sensor (with the van at operating temperature) are approximately 2285 Hz at 4.6 g/s. This value has some wiggle room and doesn’t need to be exact.
Using a Scan Tool
First, we can check the data on an OEM-level scan tool that offers manufacturer-specific live data. As we can see in the photo, our values (2512 Hz at 5.77 g/s) are close to the manufacturer’s specification (remember, there’s a little wiggle room).
You should also monitor the value while driving (with help from an assistant or recording function, of course). Typically, at WOT, the value should exceed 7 kHz.
Using an Oscilloscope
Also, you can check the MAF’s frequency output using either a digital multimeter DMM with a frequency setting or an oscilloscope. A scope is the best option since it allows you to see the MAF signal directly.
To set up the scope, you’ll need a back probe test lead to connect to the MAF signal wire (see photo). You’ll want to attach the positive (red) lead to the MAF sensor’s signal wire, and the negative (black) lead to a good ground. Consult a repair manual or repair database to determine which wire is the MAF signal wire.
Next, turn on your scope and adjust the settings as needed. Then, start the engine and monitor the results at idle.
Judging from the waveform pattern in the image below, our Astro van’s MAF appears healthy. The sensor’s signal has humps that are nice and even. Plus, the frequency of those humps (2.42K Hz) is within specification.
Snap the throttle open by hand (or have an assistant operate the accelerator pedal). If the MAF sensor is working properly, the signal’s humps should still stay nice and even, but their frequency should increase.
Checking the Output of an Analog Voltage (Hot Wire) MAF Sensor
Hot wire sensors produce an analog voltage signal that starts near zero volts. The sensor’s output voltage steadily climbs to around 5 volts as air flow through the MAF sensor increases.
To check a hot wire MAF sensor’s output, you’ll need either a regular DMM, an OEM-level scan tool, or an oscilloscope.
Our sample vehicle is a 2007 Honda Civic with a 1.8L engine and a known good MAF sensor. Because Honda does not list the MAF specifications for this car, we’ll have to use rule-of-thumb guidelines during testing.
Using a Scan Tool
The typical reading for an analog MAF sensor is around 1 volt at idle. As you can see in the image below, the sensor in our Honda is reading 1.31 volts at idle, which is just fine.
The MAF sensor signal voltage should steadily increase as the amount of air entering the engine increases. You can verify this by monitoring the value while driving (with help from an assistant or recording function, of course). Typically, at WOT, the value should be nearly 5 volts.
Using a Digital Multimeter
Good news: You can test a hot wire MAF using a traditional DMM. To set up the DMM, you’ll need a back probe test lead to connect to the MAF signal wire (see photo). You’ll want to attach the positive (red) lead to the MAF sensor’s signal wire, and the negative (black) lead to a good ground.
Consult a repair manual or repair database to determine which wire is the MAF signal wire.
Next, start the engine and monitor the results at idle. Judging from the readings in the image below, the Civic’s MAF output at idle appears fine. The reading is close to 1 volt at idle.
Open the throttle by hand (or have an assistant operate the accelerator pedal). The voltage should steadily increase as airflow increases.
Is it Really a Bad MAF Sensor?
In some instances, what seems like a bad MAF is actually a problem somewhere in the sensor’s circuit. So, if your test results point to a bad MAF, but replacing the sensor doesn’t cure the problem, you’ll need to dig a little deeper.
Consult a repair manual or a repair database for the factory wiring diagram. Then use a DMM to ensure the supply voltage and ground portions of the MAF circuit are intact. You may also need to check for continuity between the MAF sensor and the PCM.
Although somewhat rare, it’s also possible for the PCM to misinterpret the MAF signal. The dealer may have a technical service bulletin (TSB) suggesting a software update to fix the problem.
How to Clean a Mass Air Flow Sensor
In some cases, you can clean the MAF sensor to restore normal performance. You can clean the device using a dedicated cleaner that comes in an aerosol can.
What You Need to Clean a Mass Air Flow Sensor:
The tools and supplies needed to clean a MAF sensor vary, depending on what type of car you have. But, in general, you’ll need:
- Clean towels
- MAF sensor cleaner
- Safety glasses
When cleaning the MAF sensor, you should always follow the instructions listed on the product you purchased. In general, however, the process involves the following:
Warning: Do not use MAF cleaner on Karman-Vortex style MAF sensors. This type of sensor uses either an ultrasonic receiver or a pressure sensor to detect vortexes in the airflow through the sensor.
Step-by-Step Instructions for Cleaning the MAF Sensor:
1. Put on your safety glasses.
2. Locate the MAF sensor (positioned between the air filter assembly and throttle body).
3. Disconnect the MAF sensor electrical connector.
4. Use a screwdriver to loosen the clamps on the air intake duct.
5. Remove the MAF sensor by pulling it out of the air intake duct.
6. Place a clean towel under the MAF sensor.
7. Attach the straw to the aerosol can. Then spray the recommended amount of cleaner (as listed on the product instructions) onto the sensing element inside of the MAF housing.
Note: Do not allow the straw to contact the MAF sensing element.
8. Allow the MAF sensor to dry thoroughly.
9. Reinstall the MAF sensor into the air intake duct.
10. Use a screwdriver to tighten down the clamps on the air intake duct.
11. Reconnect the MAF electrical connector.
How to Replace a Bad Mass Air Flow Sensor
Sometimes, cleaning the MAF doesn’t work and you have to replace the sensor. The steps for replacement are the same as those involved in removing and reinstalling the sensor for cleaning.
Step-by-Step Instructions for Replacing a Bad MAF Sensor:
The process for your actual vehicle may vary. But replacing the MAF typically involves the following steps.
- Put on your safety glasses.
- Locate the MAF sensor (positioned between the air filter assembly and throttle body).
- Disconnect the MAF sensor electrical connector.
- Use a screwdriver to loosen the clamps on the air intake duct.
- Remove the MAF sensor by pulling it out of the air intake duct.
- Compare the new MAF sensor to the old MAF sensor to ensure that both are the same design.
- Install the new MAF into the air intake duct.
- Use a screwdriver to tighten down the clamps on the air intake duct.
- Reconnect the MAF electrical connector.
Can You Drive with a Bad Mass Air Flow Sensor?
Although in some cases, you may technically be able to continue driving with a bad MAF sensor, you should address the issue as soon as possible. A faulty MAF can damage other parts of your vehicle, such as the catalytic converter.
What’s more, some cars won’t even run when the MAF fails. In other instances, the engine will run but run poorly, making the vehicle difficult to drive.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.