A bad mass air flow sensor is often the likely culprit if your car isn't running as it should.
The symptoms of a bad mass air flow sensor include low engine power, poor fuel economy, an engine that won't start, a check engine light on the dash and a gas smell from the exhaust fumes.
The job of the mass air flow sensor is to keep track of the amount of air coming in through the air filter, so that the ECU can adjust the amount of fuel getting into the engine.
If the mass air flow sensor isn't working properly then you will be faced with one or more symptoms such as lack of power and bad fuel economy.
Of course, there are many other reasons why your car might not be running properly, so today I'm going to explain how to diagnose a bad or failing air flow meter and see what's involved in fixing it.
7 Symptoms of A Bad or Failing Mass Air Flow Sensor
As explained at the beginning of the article, the mass air flow sensor (MAF) data is vital for fuel delivery control. The amount of air entering the engine through the air filter is going to fluctuate depending on how fast the engine is running.
If you push down on the accelerator, the engine will need more fuel to increase the rate at which the pistons are moved. In order to increase speed, more fuel and air is needed. The amount of fuel must be kept in line with the amount of air coming in. This is why the mass air flow sensor is important. It must be able to accurately measure the air flow and send this information to the ECU in real time.
Therefore the most common bad mass air flow sensor symptoms usually appear in the form of an engine that is performing badly.
Here are some of the most common symptoms:
1. Check Engine Light appearing on the dashboard
A bad mass air flow sensor will usually trigger a check engine light. This is because the ECU will detect an out of range value coming from the MAF sensor and the check engine light is it's way of letting you know that something is wrong.
2. Difficulty during engine start
Since the ECU can’t measure incoming air mass, it needs to compensate for this lack of information using the rest of the engine sensors. Depending on ECU’s program sophistication and installed sensors, the vehicle start can be very difficult. On older models, you may find that the engine doesn’t start at all.
3. Poor fuel economy
Without proper information coming from the MAF, the ECU won’t be able to operate efficiently and thus, a noticeable decrease in fuel economy will be evident. Most ECUs will revert to a default air mass value range to keep the engine running. This is not an accurate value and will result in more fuel being fed into the engine than is required.
4. Rough Engine Idle
Depending on the vehicle year and maker the ECU may not compensate for this failure during idle. This might cause a rough idle or even an engine dying occasionally while idling.
5. Engine hesitation during acceleration
A bad mass air flow sensor will make very hard for the ECU incrementing the necessary amount of fuel during sudden acceleration. In such a scenario, the ECU will sense the drop in engine vacuum first and then using other sensors data will decide if more fuel is required. That process could take half a second to a second, more than enough to perceive the hesitation. This condition will be worse under wide open throttle acceleration.
6. Engine hesitation under load
Similar to the previous point, if you are driving uphill, towing, or demanding power from your engine, you will find a serious hesitation due to lack of proper information regarding air mass entering the engine.
7. Gas smell from exhaust pipe
the failure to deliver the exact amount of fuel to the engine could cause a noticeable gas smell coming from your exhaust pipe.
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What Exactly is A Mass Air Flow Sensor?
The Mass Air Flow Sensor (also known as MAF) is one of the main electronic components of the fuel delivery system. It's job is to report, in real time, the mass of air entering the engine.
This information is used by the Electronic Controller Unit (ECU) to determine the amount of fuel needed to achieve the stoichiometric ratio consisting of 14.7 parts of air per each part of fuel.
This calculation is then adjusted after analyzing the data from the rest of sensors, especially the oxygen sensor feedback that is used as a key input for the corrections of the next computing cycle. In other words, without the MAF data, the ECU would be nearly blind regarding air flow making very difficult a proper fuel delivery.
How Does A Mass Air Flow Sensor Work?
Depending on the year and make of the vehicle, you may find different types of mass air flow sensors, from the old-style vane air flow meter to the modern era hot-wire mass air flow meter.
Since hot-wire technology is the most widely used today, we'll assume that your MAF is using that design.
The name “hot-wire” is a direct reference to the sensor design where a thin filament is heated to a constant temperature by an electronic controller. The filament is usually inside a cavity that is part of the MAF sensor.
The theory behind this technology is quite simple, the amount of current required to keep the filament at its preset temperature is directly proportional to the air volume entering the engine. The sensor’s integrated controller is in charge of monitoring that current and then report it to the ECU.
There are variations of hot-wire technology too. As you might notice, the hot-wire sensors don’t measure “mass” but incoming air “volume”. In order to know the air mass, you do need air volume but also air temperature and barometric pressure. That’s why some manufacturers include an Intake Air Temperature sensor (IAT) and barometric sensor (BARO) into the MAF.
The combination of the three sensors provides the necessary information for calculating the air mass flow in real time. Some mass air flow sensors literally integrate the IAT and BARO into one single part, while others just use the IAT and BARO information to calculate the mass keeping each sensor independent.
Another important factor to consider is the MAF signal output. The main difference between an analog mass air flow sensor and a digital one is its output.
A digital output is instantly interpreted by the ECU without the need of passing the signal through ECU’s analog-to-digital converter (ADC). That function is delegated to the MAFs own ADC which is a component of its integrated controller.
Summing up, before going any further you should understand the following key concepts:
- Most mass air flow sensors use hot-wire technology.
- You may find MAF designs with integrated IAT and sometimes BARO sensors. (Keep that in mind during diagnostics)
- Some air flow meters have digital outputs. Digital outputs can only be measured using frequency meters (DVOM or oscilloscope) or an automotive scan tool.
How Do You Diagnose A Failing Mass Air Flow Sensor?
For the purpose of this article, it’s assumed that you have a basic knowledge of safety precautions while working on your vehicle.
Depending on the mass air flow sensor installed on your vehicle, you may need one or more of the following tools:
- OBD2 Code Reader
- Digital multimeter (DMM)
- Automotive diagnostic scanner with live data capabilities
- Digital Oscilloscope
A professional grade automotive scanner with live data graphing capabilities is not essential for testing the MAF sensor but certainly would speed up the process no matter what kind of sensor is installed.
Always refer to the appropriate OEM literature when possible. Original manufacturer diagnostic procedures should always have precedence over a generic workflow.
That said, let’s start the diagnostic process!
1. Preliminary Steps
Recommended tools: OBD2 Code Reader / automotive scan tool
> Data trouble codes: using an OBD2 code reader take note of any DTC present in the ECU memory. For doing so you only need to connect your code reader and turn the ignition key on (engine off), a state usually known as KOEO. Common error codes include P0299 (if turbocharger is fitted), P0300, P0101, P0102 and P0171.
> Data trouble codes clear: now that you know what codes were stored into memory you need to clear them. This is usually done selecting “clear codes” in your tool.
> Driving cycle: disconnect your OBD2 code reader and start the engine, this state is commonly known as KOER (Key On Engine Running). If the Check Engine Light turns on then stop the engine and continue with the diagnostic. If the Check Engine Light remains off then drive the vehicle for 5 to 10 minutes. If the light remains off you may have an intermittent problem. If the light lit during your driving cycle then continue with the diagnostic process.
IMPORTANT: The following tests assume that the MAF sensor harness is already unplugged, refer to “how to replace” section if you have any doubt.
2. Visual Inspection
Recommended tools: none
> Sensor wiring: perform a meticulous visual inspection of MAF sensor wiring. Look for burnt, damaged, corroded or deteriorated wires. Pay special attention to possible shorts to ground or open circuits produced by a faulty insulation.
> Sensor harness connector: similar than before, perform an exhaustive visual inspection of MAF sensor connector. Ensure that the ignition key is switched off and unplug the MAF sensor harness connector. Look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.
> Sensor internal condition: it’s very common finding MAF sensors reporting false values due to a clogged or dirty hot-wire cavity.
Air entering the engine may have oil particles as well as dust and other pollutants that decrease the MAF sensibility over time. An exhaustive visual inspection to discard this condition is a must before going any further. You will need to remove the MAF sensor from the vehicle, refer to the last section to learn how to do it.
Once you have removed the sensor you can start cleaning its internal conduits. Please notice that hot-wire sensors are very delicate, you need to proceed with extreme caution and never ever use any decarbonizing, acid, aggressive detergent or similar product on it.
In order to clean the sensor, the best method is using a dedicated Mass Air Flow Sensor cleaner (MAF cleaner) or a biodegradable liquid soap with abundant water and Q-tips type cotton swabs. Re-install the sensor once you finished this maintenance.
Fix any wiring problem before continuing.
3. Electrical Tests
Recommended tools: digital multimeter (DMM)
> Reference voltage: as mentioned earlier in the article, many designs include IAT and sometimes even BARO within the MAF. Refer to the appropriate wiring diagram to identify your connector signals.
The intake air temperature sensor is analog, meaning it needs a reference voltage (of 5V) coming from the ECU. In order to measure the reference voltage, you will need to enter KOEO mode (Key/Ignition On - Engine Off) and turn the digital multimeter dial to “voltage” test mode. Now place one multimeter probe in a known ground (battery negative is a good one) and the other probe tip in the terminal of the MAF sensor wiring harness corresponding to the ECU reference.
If there is no voltage (or is below 4V) then you must check the continuity and electrical resistance of the wire from the terminal all the way to the ECU itself. In case your MAF integrates a BARO sensor repeat the same procedure.
> Ground test: this test assumes that you found a good reference voltage coming from the ECU. Once again, enter in KOEO mode, turn the digital multimeter dial to “voltage” test mode and place one probe tip on the MAF harness terminal corresponding to the reference voltage and the other on the terminal that should be the ground.
It’s not unusual to have more than one ground coming from the ECU, especially on integrated designs. Ideally, the voltage would be the same value as the previous test, if the difference is greater than 10% then you will need to check electrical resistance and continuity of the ground line from the MAF harness all the way down to the ECU. Repeat this test for each of the MAF ground signals.
> Supply voltage: as explained in this article, the MAF sensor uses a hot-wire design. The integrated MAF controller uses a 12V supply voltage to power itself but also to operate the hot-wire filament. Without this power line, the MAF would shut-off immediately. Refer to the proper wiring diagram and identify the terminal corresponding to the +12V line.
In order to measure the supply voltage, you will need to enter in KOEO mode and turn the digital multimeter dial to “voltage” test mode. Now place one multimeter probe in a known ground (use battery negative again) and the other probe tip in the terminal of the MAF sensor wiring harness corresponding to the +12V supply. If the measured value is below 95% of the battery voltage then you will need to check the entire line from the MAF harness all the way to the ECU itself.
4. Scan Tool Tests
Recommended tools: automotive scan tool with live data capabilities.
Optional tools: a professional automotive diagnostic scanner with live data graphing ability.
> Mass Air Flow Sensor live test: using an automotive diagnostic scanner you can perform a MAF “live test”. First, enter in KOEO mode and lock the MAF sensor related data on the display. Depending on your vehicle and scan tool maker you may have several items to check. Focus on the calculated MAF output, many times expressed in grams per second. You could also lock on screen the MAF output voltage if you can’t find its calculated value. Once you are ready start the engine to enter KOER mode. If your scan tool has a graphing ability turn it on. Graph engine RPM and MAF output at the same time. Gently accelerate and decelerate the engine. The resulting lines should be similar, increasing and decreasing simultaneously. The greater the difference between them the greater the probability that you have a bad mass air flow sensor.
How To Replace A Bad Mass Air Flow Sensor
The MAF sensor is usually located between the throttle body and the air filter housing.
Recommended tools: OBD2 Code Reader, safety glasses, mechanics gloves, appropriate screwdriver and/or wrench, proper illumination (LED flashlight),
- > Preliminary steps: ensure that the ignition switch is off and then disconnect the vehicle battery. Detaching the negative terminal is usually enough.
- > Remove MAF sensor connector: this step may be tricky in some models. Generally speaking, the connector has a plastic tab that you should push down while you pull out the harness. But in many cases, there is a small security clip (sometimes plastic, sometimes metallic) that must be removed first using an adequate screwdriver.
- > Remove MAF sensor screws: most mass air flow sensors use clamps on each side to connect to intake manifold air duct. Loose both clamps using the adequate screwdriver and remove the sensor. Be cautious while doing so.
- > Install the new MAF sensor: retire the old sensor and then install the new one. Double check that you’re using the adequate part number.
- > Reconnect sensor and battery: once you are done, reconnect the sensor (don’t forget security clips) and then the battery terminal.
- > Clear DTC memory: using the OBD2 code reader clear the ECU memory to prevent any false code been stored. You will need entering in KOEO mode for doing so.
- > Perform driving cycle: once you are done, drive the vehicle for more than 10 minutes. Try to accelerate as you would normally do.
The symptoms of a bad mass air flow sensor include a car that is difficult to start, bad fuel economy, a rough idle, lack of power especially when accelerating and sometimes a fuel smell from the exhaust fumes.
A properly functioning MAF sensor is vital for an engine to run efficiently. If you suspect your car has a bad MAF sensor then you should check it out as soon as possible.