For an engine to run most efficiently and supply the catalytic converter with the proper gasses, it can convert to non-harmful emissions, the engine’s air-fuel ratio must run as close to the proper stoichiometric ratio as possible. This means that for every one pound of fuel that is consumed in a gasoline engine, 14.7 pounds of air are needed for perfect combustion.
For years, the carburetor made this calculation, albeit inefficiently, but it worked. As emission controls and computing technology evolved, better ways of keeping the proper air-fuel ratio were developed.
The Powertrain Control Module (PCM) of today’s vehicles needs to know exactly how much air the engine is consuming at any given moment so that it can calculate the amount of fuel to add to achieve the optimum air/fuel ratio. There have been several methods developed to do this, but the two most common methods are speed density and mass airflow.
Two methods:
The speed density system determines the amount of air entering the engine by performing a complex calculation. The PCM will use information from the manifold absolute pressure sensor (MAP), intake air temperature sensor (IAT), coolant temperature sensor (ECT), and engine RPM to calculate the amount of air the engine is consuming. The PCM will then process this sensor information and cross-reference it to a predetermined fuel map stored in the PCM to determine how much fuel the engine requires.
The mass airflow system will use a Mass Air Flow sensor (MAF) installed between the air filter housing and the throttle plate to precisely measure the amount of air entering the engine. This direct measurement of the air entering the engine makes the mass airflow system more accurate than the calculated airflow of the speed density system, and that is why the MAF has been adapted and used by almost all manufacturers today.
MAF ... how it works
There are two common MAF sensor designs: hot wire or sensing film. They function similarly but differ by the type of sensing device that is placed into the engine’s incoming airstream.
Each MAF sensor will ordinarily have 5 wires connecting it to the vehicle’s wiring harness. There will be an ignition circuit (12V), a low reference (sensor ground or chassis ground), a MAF signal circuit, a 5V reference circuit and an IAT (intake air temperature) circuit. Almost all MAF sensors will have an integrated IAT sensor, simplifying engine sensor installation and providing a more accurate air temperature reading to the PCM. There is an 8 wire multifunction Intake Air Sensor used on many GM products now, but more on that in a bit.
The hot wire design uses 2 sensing wires generally made of platinum: 1 hot and 1 cold. The MAF sensor circuitry will apply current to the hot wire, heating it up so that it is typically 392°F (200°C) hotter than the cold wire. The MAF sensor’s objective is to maintain this temperature difference.
The hot and cold wires of the MAF sensor are exposed to a portion of the air flowing into the engine. As this air passes over the hot wire it causes it to cool. The more air flowing over it, the more it cools, and the more current is required to maintain the temperature differential.
Hotwire MAF sensors are susceptible to contamination, and they are physically delicate. Some of the newer hot wire sensors encase the hot wire in glass to protect it from deposits and use unique housing designs to lessen the sensor wire’s exposure to debris from the incoming air stream.
MAF sensors that use a film, hot-film, or thick film (they use different names but are essentially the same) sensing component will function the same way, but instead of using two wires, the hot-film sensor will use a centrally heated film or metallic grid-type element. One side of the film will be exposed to the incoming air while the backside of the film is shielded and maintains a lower consistent temperature. Film sensors usually have a lower temperature differential than hot wire. The current differential between the two sides is measured and then that information is sent to the PCM, so it knows the mass of the incoming air. Film type MAF sensors tend to be more robust and are less susceptible to the contamination of the hot wire type.
The MAF sensor will send the PCM information about the amount of current that is being used to maintain the temperature differential between the hot and cold wires. The signal from the MAF sensor to the PCM can be voltage or Hz but it’s this signal that the PCM will use to calculate the amount of incoming air into the engine.
Both of the MAF sensor designs work using the same principle. They are measuring the cooling effect of the incoming air flowing into the engine by monitoring electrical current changes. The MAF output signal sent to the PCM will be proportional to the sensed airflow according to this chart:
The signal sent from the MAF to the PCM will be manufacturer-specific, but the function and results are the same whether the signal is Hz or volt.
What can go wrong with the MAF system
The PCM is depending on the MAF to report all the air that is going into the engine so it can deliver the proper amount of fuel. Any air that is entering the engine after the MAF or unmeasured air or false air will cause a low MAF signal and normally result in a (lean) under-fueling condition. Cracked intake ducting, disconnected PCV breather hoses, intake/vacuum leaks and other conditions can cause this situation. The results are often low-speed stalling, SES light or rough idle.
MAF sensor contamination is also a major issue. Any debris that gets past the air filter and comes in contact with the wire or film sensing element can get baked on, effectively insulating the sensor. Over-oiled permanent (reusable) air filters, poor quality air filter elements, loose hard plastic inlet pipes, MAF sensor design and engine oil from the PCV are common causes of MAF contamination.
The opposite effect can happen if something becomes lodged or stuck onto the sensing wire or film. If an insect or dandelion seed for example, gets wedged against the sensing wire or film, it can absorb part of the heat and the MAF will then overestimate the incoming airflow, causing a rich mixture, creating an over-fueling condition and possibly setting rich fuel trim codes (P0172/P0174).
Because the MAF was designed to sample only a small part of the actual intake airflow, the system can only assume that it is installed in a factory-built, factory-calibrated environment. This means that any changes to the airbox, ducting, MAF housing or location, MAF screen, etc. may cause undesirable effects because of turbulent airflow that the MAF wasn’t designed to see. The results could be undesirable engine performance, SES light and fuel trim conditions.
MAF sensors do not have the ability to detect a torn intake duct, a worn camshaft, skipped timing chain or timing belt, an EVAP fault or plugged exhaust that may be affecting the base engine’s ability to take in the required amount of air for proper engine operation.
These scenarios can result in the MAF sending the PCM an erroneous signal to the PCM, when in fact there may be nothing wrong with the MAF at all. It is important to remember the base engine must be working as designed before a MAF sensor is blamed as the root cause of the issue.
Common issues, pattern failures and diagnostic tips
A partially restricted exhaust can set MAF code P0101 (Mass Airflow Circuit Operating Range) without setting any fuel trim codes. The reason for this is the MAF output is low because the airflow is low due to the restricted exhaust, so the fuel trims will be normal. The engine will have poor power, the PCM will see lower airflow for a given throttle position and blame the MAF for the problem.
MAF codes and fuel trim codes that are set together are usually an indication of a bad or contaminated MAF. MAF codes and O2 codes set together usually point to a bad O2 sensor.
A MAF can be cleaned, gently, but remember to allow the sensor to cool first and use the appropriate electronics cleaner. Cleaning a MAF won’t fix a glitch or intermittent dropping out MAF signal condition.
Early 2000’s Toyotas used a hot wire MAF that was prone to contamination. This would start off as a fuel trim issue and manifest from there. Because of the contamination coating and insulating the hot wire sensor, the MAF would under-report the amount of fuel needed, creating a lean condition. The fuel trims would try to compensate, but once they hit +35% on LT and ST, the check engine light would come on and the P0171 lean code would appear. Not very often would a MAF code be set. The first step in diagnosing was to remove the MAF and look at the sensor wires. They can often look like a fuzzy caterpillar with so much debris stuck on them. A gentle cleaning would remove most of the debris and restore the performance, but a replacement MAF was the best answer after verifying the concern. Cheaper replacement air filters are often the cause of such contamination from loose filter fibers attaching to the hot wire sensing element.
Late-model Nissan Altima’s are prone to setting a P0101 MAF code often with no driveability concerns. Nissan has issued a TSB asking the tech to inspect the PCV and update the PCM. This worked some of the time, but the MAF sensor or a corroded MAF sensor connector was often the actual issue. Replacing the MAF with a factory one, combined with the PCM and PCV update was often the only way to solve and repair the P0101 code. There is a specific idle air volume relearn procedure that must be carefully followed as well, but the most important part of the repair was to use a factory Nissan MAF sensor.
In 2013, GM started to use an 8 wire MAF. They even changed the name of the sensor to the “Multifunction Intake Air Sensor.” This sensor still works and functions as a normal hot wire MAF but now there are extra sensors integrated into the unit. The multifunction intake air sensor now has a barometer sensor, a humidity sensor and an extra IAT sensor (IAT2). This sensor allows the PCM to compensate for humid weather conditions that could lead to a rich air-fuel ratio (water vapor displaces O2 molecules) and the barometer to adjust for higher altitudes, less O2 and a rich mixture. The sensor output for IAT2 and the humidity sensor share a common wire and the signal is sent as a frequency for IAT2 and a duty cycle for the humidity.
The MAF was developed to be a fast and efficient device that will accurately measure the amount of air going into the engine so the PCM and other sensors can keep the air-fuel ratio as close to perfect as possible. The advances in electronics and computing power have allowed this to happen. Today’s MAF sensors are robust, resist contamination and aren’t prone to failure like their predecessors.
