P0101 - Mass Air Flow Sensor Circuit Range/Performance

The mass airflow sensor (MAF) measures the amount of air entering the engine. The OBD-II code P0101 is set when the signal from the mass air flow sensor is out of expected range, or in other words, the MAF sensor doesn't work properly. In most cases, this code is not very difficult to diagnose.

• Symptoms
• Causes
• How the code P0101 is diagnosed
• Examples
• Common problems in different cars
• How the code P0101 is set

Symptoms:

The engine computer uses the mass air flow sensor signal to determine how much fuel to inject. If the sensor doesn't measure the air flow correctly, the fuel injection system won't work properly. Common symptoms include:
- Stalling
- Difficulty starting
- The engine starts and stalls
- Lack of power
- Unstable idle
Because the mass air flow sensor readings are used to calculate the engine load, the traction control system may stop working and the traction control warning light might also come on on the dash. For the same reason, the automatic transmission shifting pattern may also change.

Causes:

The most common causes of the code P0101 are:
- Dirty, obstructed or faulty mass air flow sensor (MAF)
- Cracked, ripped or disconnected intake boot (snorkel)
- Vacuum leaks (from stuck open PCV valve, leaking intake gaskets, etc.)
- Clogged up or improperly installed air filter
- The mass air flow sensor element contaminated by excess oil from an oil-soaked air filter
- Clogged-up catalytic converter or restricted exhaust
Other possible causes include:
- Dirty throttle body
- Bad or dirty manifold absolute pressure sensor (MAP)
- Electrical problems with the MAF sensor connector or wiring
- Stuck open EGR valve
- Incorrect valve timing
- faulty engine computer (ECM)

How the code P0101 is diagnosed:

First the mass air flow sensor must be checked and cleaned if dirty. Sometimes a foreign object like a leaf or dust can clog-up the airflow sensor measuring element (hot wire) causing this code. See the photo of a clogged up mass airflow sensor below.

If the problem started soon after the air filter was replaced, the air filter installation as well as the air flow sensor must be checked.

In some cars (e.g. Volkswagen) a metal mesh intake screen installed before the air filter can be plugged up too. It must be checked and cleaned.

The intake boot must be inspected for cracks and rips. This problem is common in many European vehicles, e.g. BMW, Volvo. Other possible sources of vacuum leaks must be inspected.

The mass air flow can be tested with a scan tool that displays the live data. The airflow sensor readings at different rpms (e.g. 1000, 2000, 3000) must be compared to the specifications or to the readings of a known good sensor. A faulty airflow sensor is very common in many cars (e.g. BMW, VW, Mercedes-Benz, GM)

In some vehicles (e.g. Chevy Tahoe, Silverado) the code P0101 can be caused by a clogged catalytic converter. To diagnose this issue, exhaust back pressure or the engine vacuum must be checked. Read more about symptoms of a clogged catalytic converter further.

Examples of what can cause the code P0101

Often the code P0101 is caused by a vacuum leaks. A vacuum leak allows "unmetered air" to enter the engine by-passing the airflow sensor. In the first photo, you can see a cracked intake boot. The amount of air entering the boot through this crack is not measured by the sensor, therefore the airflow sensor reads a much lower amount of air flow than actually is. One of the symptoms of a vacuum leak is the hissing noise from under the hood when the engine is idling. In the second photo, you can see a chunk of a leaf that blocks the sensor wire. Because of that, the sensor cannot read the airflow properly and sets the code P0101.

Cracked intake boot Cracked intake hose