How to Check and Change a Mass Air Flow Sensor for a 1999 Subaru Forester

This tutorial walks you through how I checked and changed my Mass Air Flow sensor using the tools available at TechShop San Francisco.  People usually say "I made it at TechShop!"  But I guess this really is more of a "I fixed it at TechShop!"

Tools you will need:
Security Torx Screwdriver set
Flathead Screwdriver
Multimeter (I used Techshop's Rigol DM3058 Multimeter, but a handheld will work just fine).
Mass Air Flow Cleaner (available at your local autoparts store)

The Mass Air Flow Sensor sits within your cars air hose and lets the cars computer know how much air is flowing into the engine.  This allows the computer to adjust to achieve an appropriate ratio of air and fuel (and spark) in a fuel injection engine.

On a 1999 Subaru Forester, the MAF Sensor is located on the passenger side, on the air hose that comes off of the air filter.  The one in the picture below has a blue stripe on the top.

1) Undo the metal clasps on the Air Filter Cover (rectangular plastic box shape abbutting the passenger side.
2) Lift up on the cover.

There is a plug that attaches to the MAF Sensor.  This is what connects the sensor to the computer.  Unplug it by pressing down on the tab on the top and wiggling it until it comes free.

The MAF sensor attaches to the air intake hose that runs between the air filter and your car's air intake using two security torx screws.  Unscrew the torx screws using the security torx screwdriver.  These screws are similar to regular torx screws except that they also have a small cylinder shaped nub sticking up out of the middle.  A security torx bit has a recess to fit over this nub.  You can find a set of these for cheap (under $10 at an autoparts store).  I just borrowed one from TechShop. Use the flathead screwdriver to help pry the MAF free of the housing a bit.

The MAF Sensor uses a wire (in my case it is a platinum wire) to gauge the amount of air that is flowing past the filter.  This wire is heated to 210 degrees above the surrounding air temperature using a current.  As air flows over the wire, it cools and the car's computer senses a change in the amount of current needed to maintain the 210 degree difference.  The computer uses this information to calculate the volume of air flowing into the engine and to adjust the mix of air and fuel accordingly.

If this wire gets dirty or corroded, its resistance will shift out of spec and the computer won't have an accurate gauge on the volume of air flow.

First, clean the wire using some Mass Air Flow Cleaner (available at your local autoparts store).  Simply spray about ten bursts onto the wire (hidden in the end of the MAF Sensor) and allow it to air dry.  DO NOT WIPE.  Wiping the wire may contaminate it.

Use Multimeter to check the terminals that plug into the MAF Sensor Plug. (they look like four metal tabs)

You are testing the resistance of these terminals to see if they are within the appropriate range.

Terminals 1, 2 and 4 should have approximately 1 mega-ohm of resistance to ground.  Terminal 3 should have zero ohms resistance to ground.

If any of the terminals are outside of this range, you will want to replace the MAF Sensor.

The sensor itself costs about $50-$70 to replace.  However, they may try to sell you the whole housing etc... which will run you more than $350. 

Place into housing so that the wire is facing into the air flow (toward the air filter). 

Screw into place using the security torx screws.

Plug in MAF Sensor Plug.

Close and latch air filter housing.

You're done!