P0101 - OBD-II Trouble Code

Auto Systems and Repair

Mass Air Flow Sensor Circuit/Performance Malfunction

Our emissions expert has put together the following information about the P0101 fault code. We have also included diagnostic procedures you can take to your repair shop if the mechanic is having difficulty analyzing the code.

OBD II Fault Code

  • OBD II P0101

Fault Code Definition

  • Mass Air Flow Sensor Circuit/Performance Malfunction
What does this mean?
The Mass Airflow Sensor (MAF) measures the amount of air entering the engine. This is important information the Engine Control Module (ECM) uses to calculate the proper amount of fuel for combustion and the amount of ignition timing advance.

Code P0101 is set when this sensor's output becomes irrational and/or out of range.

Symptoms

  • Check Engine Light will illuminate
  • In some cases, there may be no adverse conditions noticed by the driver
  • In other cases, there may performance problems, such as a lack of power on acceleration, "coughing," misfiring, and/or backfiring
  • Problems idling
  • Black smoke from the tail pipe and poor fuel mileage
  • Other codes may be triggered by the Mass Air Flow Sensor problem such as Oxygen Sensor and/or Fuel Trim Lean/Rich codes (P0130/P0136, P0131/P0137, P0132/P0138, P0135/P0155, P0171/P0174, P0172/P0175)

Common Problems

  • Large vacuum leaks, split Intake Air Boot or PCV Hose, defective intake manifold gaskets
  • Mass Airflow Sensor (MAF)
  • Mass Air Flow Sensor circuit and or wiring problems
  • Defective Barometric Pressure Sensor
  • Dirty or contaminated Mass Air Flow Sensing wire or filament
  • PCM software needs to be updated

Common Misdiagnoses

  • Oxygen Sensors
  • Mass Air Flow Sensor
  • Powertrain Control Module (PCM)

Polluting Gases Expelled

  • HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog
  • CO (Carbon Monoxide): Partially burned fuel that is an odorless and deadly poisonous gas
  • NOX (Oxides of Nitrogen): One of the two ingredients that, when exposed to sunlight, cause smog

Want to Learn More?
The purpose of the Mass Air Flow Sensor is to measure the amount of air entering the engine. The units of measurement are typically expressed in grams per second or GPS. The most commonly used Mass Air Flow Sensor employs a precisely heated wire that spans a vortex in front of the Throttle Body on the Intake Manifold. The PCM constantly adjusts the amperage to this Mass Air Flow Sensing wire in order to maintain a temperature that is precisely 100 degrees above the Intake Air Temperature. As intake air passes over this wire, it is cooled and within a few milliseconds, the PCM  heats this wire back up to 100 degrees above the Intake Air Temperature Sensor reading.

The amount of amperage needed to maintain the Mass Air Flow Sensing Wire at this 100 degree level is converted by the PCM, using an algorithm, into a precise measurement (in GPS) of the amount of air entering the engine. The Powertrain Control Module then takes this information and calculates the amount of fuel for the injectors to mix with the incoming air charge in order to optimize combustion in the cylinders for the most power, while still maintaining maximum fuel economy and the lowest possible emissions. The PCM also uses the Mass Air Flow Sensor readings to influence the amount of Spark Advance to apply to each cylinder before it fires.

A few manufacturers install an Ultra Sonic Mass Air Flow Sensor that uses the changes of a sound wave inside a vortex to measure the amount of air coming into an engine. Some of the very old Mass Air Flow Sensors use a vane-like door for measurement. As the door gets pushed farther open by increases in the amount of intake air entering the engine, the movement gets translated into a rising voltage that the PCM converts into the GPS metric.

P0101 Diagnostic Theory for Shops and Technicians
When the code P0101 is set, record the freeze frame data in fine detail. Then duplicate the code setting conditions on a test drive, paying particular attention to the GPS (or kilograms per hour) readings, load, MPH, and RPM. The best tool to use on this test drive is a data streaming scan tool that has factory quality and dedicated live data. Be sure to verify the code conditions before you advance to the next set of tests.

If You Can Verify the Code Setting Malfunction
If you can verify the code setting malfunction, then do a very careful visual inspection of the sensor and the connections. Visually inspect the Intake Air Boot for any signs of tears or cracking—you may have to pull on it to expose all of the sections. Also, do a very careful visual inspection of the PCV hoses and test the integrity of the Intake Manifold and its gaskets with a fuel substitute such as Propane gas. Verify that there is a 12-volt signal and a good ground to the sensor and that they meet the required times, per the manufacturer diagnostic documentation.

If these inspections don't diagnose the problem, then replace the Mass Air Flow Sensor with an OEM/OEM rebuilt unit. Aftermarket new or rebuilt Mass Air Flow Sensors are very inconsistent and often make the vehicle run worse and/or fail in ways that may be difficult to diagnose. The OEM units may not work properly either—some can fail as early as 100 miles.

If You Cannot Verify the Code Setting Malfunction
If you cannot verify the code setting malfunction, then do a very careful visual inspection of the sensor and the connections. Visually inspect the Intake Air Boot for any signs of tears or cracking, you may have to pull on it to expose all of the sections. Also, do a very careful visual inspection of the PCV hoses and test the integrity of the Intake Manifold and its gaskets with a fuel substitute such as Propane gas. Verify that there is a 12-volt signal and a good ground to the sensor and that they meet the required times, per the manufacturer diagnostic documentation.

Watch the data stream of the Mass Air Flow Sensor while the engine idles and then slowly and carefully raise the engine RPM. The grams per second should slowly and smoothly rise. At idle and in "Park," the GPS should be around three to five. Next, drive the vehicle and do a Wide Open Throttle test (WOT) under maximum load. The GPS should go as high as 150 to 200, depending on the number of cylinders and displacement of the engine.

If the data stream test does not produce any conclusive results, then connect a lab scope to the signal wire with the voltage set at 20 and the time divisions at 100 milliseconds. Snap the throttle open and watch the signal. Do the same no-load idle test and WOT test at maximum load, described in the section above. The signal trace on lab scope should spike up to its maximum voltage without any "sharks' teeth," glitches, or drop outs in the quality of the signal. if you still cannot find the problem, here are some approaches to deploy next.

  • If you can receive authorization from the customer to keep the vehicle overnight, clear the code and test drive the vehicle by driving it home and then back to work in the morning, making sure that you are duplicating the code setting driving conditions on both trips. If the code still does not come back, you can give the customer the option of replacing the Oxygen Sensor as a diagnostic step since the sensor is the most likely problem and the code will presumably set again. If the customer declines, then return the vehicle with a clear description of the inspections and your findings plainly attached to the final copy of the repair order. Keep another copy for your own records in case you have to re-visit this inspection for any reason.

  • If this is an inspection for an emissions failure, most government programs suggest that you replace the sensor as a preventative measure so the vehicle won't remain in a highly polluting operational condition. After the Oxygen Sensor is replaced, the monitors will have to be re-set and this, too, will test most phases of the Oxygen Sensor system to ensure that the problem was solved. Be sure to verify that the Mode 6 test IDs and component IDs that pertain to fuel control are well within the parameter limits. If there is a problem with re-setting the monitors, continue the inspection until you find the root cause of the problem.

An "under reporting" Mass Air Flow Sensor can be a common cause of a code P0101. Essentially, this means that the Air Flow Sensor is telling the computer that much less air is entering the engine than actually is.

Since the oxygen sensors are telling the computer that more fuel is needed, this causes confusion in the computer because the Mass Air Flow Sensor is still saying there is too little air and the Oxygen Sensor is reporting that the mixture is still too lean. The computer tried to compensate, but since resolution is impossible, it sets the code. It is important to restate that the Oxygen Sensors are accurate—the fuel mixture is too lean. In this case, the Air Flow Meter or Sensor is inaccurately reporting the real amount of air entering the engine.

  • There is a very effective "truth test" for any Mass Air Flow Sensor. Start the engine, let it idle, and then check the Barometric Pressure reading on the scan tool data. If the reading is about 26.5 Hg and you are close to sea level, you know that you have a defective Air Flow Meter because it is telling you that you are at about 4500 feet above sea level. (These conversion tables will help.) The Barometric Pressure Sensor is part of the Mass Air Flow Sensor and will cause the Mass Air Flow Sensor to send incorrect data to the engine control module.

  • Sometimes the Air Flow Sensor and the sensing wire get covered with dirt, dust, or oil residue, which can also set a P0101. Cleaning the sensor might hold off problems for a while, but eventually, the MAF sensor should be replaced. Always make sure the Air Filter and its enclosure are dirt-, dust-, and oil-free. If you clean and replace the filter and its enclosure as needed, you will prevent the new MAF from failing.
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