The mass air flow sensor or MAF sensor is part of the intake system in Subaru Legacy responsible for measuring the amount of air entering the engine. Driving your Legacy with a bad MAF sensor can damage the engine, so you must act urgently to avoid more expensive repairs.
The most common signs of bad mass air flow sensor (MAF) in Subaru Legacy are loss of power or slow acceleration, engine hesitation or jerking during acceleration, erratic idling, misfires, poor exhaust emission values and sometimes black smoke comes out of the tailpipe.
The mass air flow sensor is part of the engine’s intake system and is located between the air filter and the throttle body. It is not only important for power delivery in your Legacy, but also for the lowest possible pollutant emissions.
What happens when MAF sensor malfunctions in Subaru Legacy?
If the MAF sensor is defective or dirty, it does not send correct measured values to the control unit of Legacy. The optimal amount of fuel can no longer be calculated which results in either too little or too much fuel injected into the combustion chambers. The engine runs too “lean” or too “rich”. This can cause the engine warning light to illuminate in the instrument cluster and the fault can be read out by a diagnostic device.
What causes MAF sensor to fail in Legacy?
The most common cause of a malfunctioning MAF sensor in Subaru Legacy is dirt and grime accumulation on the sensor. Air filters are effective at cleaning the air to a certain extent. When the vehicle is running, dirt and other airborne particles that escape from the air filter can settle on the sensor. And over time when enough dust accumulates, the sensor starts to give erroneous readings.
Too many miles on the sensor
The mass air flow sensor is a wearing part. As the mileage increases on your Legacy, it can happen that increasingly inaccurate values are sent to the engine control unit. With a noticeable loss of performance and high mileage, a defective MAF sensor should definitely be considered. There is no fixed timeline for when MAF sensors fail, sometimes they can last the lifetime of the vehicle, other times than can fail even before the warranty expires.
When driving fast in heavy rain, water can occasionally pass through the air filter and can damage or contaminate the MAF sensor, which can severely impair its functionality.
In rare cases, the MAF sensor can also be damaged by oil vapors flowing back out of the crankcase ventilation after the engine has been switched off.
Bad connector or electrical wires
In rare cases, it is possible that the fault codes or the symptoms you’re seeing in your Legacy are not due to a bad MAF sensor, but because of broken electrical wires or a bad connector. This can especially happen in older vehicles or in case of rodent damage.
How to check if the MAF sensor is bad in Subaru Legacy?
Reading out the fault codes using a diagnostic device is relatively reliable. If you see P0100, P0101, P0102, P0103 or P0104 code in the OBD2 scanner, it indicates a problem with the mass air flow sensor. However, it can happen that there are no error codes. In case of doubt, the sensor may have to be replaced. But this should only be done after an extensive analysis as the MAF sensor may not be responsible for the symptoms of poor acceleration in your Legacy.
A more accurate diagnosis can be performed by analyzing the MAF sensor Parameter Identification Data (PID) information on a scan tool. However, this should be best left to trained professional mechanics, as it requires reading and interpreting the measured values from the MAF sensor.
Cleaning dirty MAF sensor in Subaru Legacy
Cleaning the mass air flow sensor can help the sensor measure the correct values again, but only if the erroneous readings are caused by dirt accumulation. However, caution is advised here, as the sensor parts are very sensitive to touch.
- Find the location of the MAF sensor in your Legacy, it will be somewhere between the air filter box and the throttle body.
- Unplug the electrical connector.
- Remove the sensor from the housing.
- Spray at least 10 spurts of MAF cleaner onto the sensor and then let it air dry for an hour or until completely dry.
- Put everything back together and start the engine.
Note: Do not use a cloth or cotton swabs to clean the sensor. Also, do not use anything other than the MAF cleaner to clean the sensor, otherwise the sensor may get damaged.
Use OBD2 scanner for diagnosis
Since Subaru Legacy is equipped with on-board diagnostics (OBD), a fault diagnosis can provide initial indications of where the malfunction is located.
To begin troubleshooting, you must first connect the diagnostic tool to your Legacy. The OBDII connector is usually located under the dashboard. With the tool connected, turn on the ignition. Most diagnostic devices then ask for some information about the vehicle. It is important that you enter this 100% correctly, otherwise the result of the search may be inaccurate. In addition to the vehicle make, model, and engine type, you usually also have to type in the Vehicle Identification Number (VIN). Since some OBD codes are manufacturer-specific, the scanner will be able to give you more accurate information if you enter more details about your Legacy.
Diagnostic error codes for MAF sensor
P0100 = Mass or Volume Air Flow Circuit Malfunction
P0101 = Mass or Volume Air Flow Circuit Range/Performance Problem
P0102 = Mass or Volume Air Flow Circuit Low Input
P0103 = Mass or Volume Air Flow Circuit High Input
P0104 = Mass or Volume Air Flow Circuit Intermittent
There are many reasons why MAF sensor is malfunctioning in your Subaru Legacy. When looking for the reason, you should always start with the most obvious cause, dirty MAF sensor that only needs cleaning.
In any case, it is advisable for laypersons to visit a workshop. A professional mechanic can swiftly diagnose the problem for you.