MAF Sensor Diagnostics

How it works
The MAF or Mass Air Flow sensor has become a standard part on most
engine applications, and is mounted in the intake air stream just past the air
filter. It measures the amount of air (volume) of the intake flow by way of
a thin wire mounted in the air stream. This wire is heated by the MAF with
a pre-calculated voltage/current value. The heat generated by the hot wire
is reduced as the intake air flows around it. The more air, the greater the
heat loss.

Therefore, the electric current supplied to hot wire is changed by the
temperature of the hot wire as air flow increases. The ECM detects this
air flow change by means of this current and voltage changes.

Symptoms of a faulty MAF
Poor idle, loss of performance, or sluggish performance and even stalling
are associated with a failing MAF sensor.  An increase in SFT
(Short Fuel Trim) percentages is another indicator.  Sometimes even a
decrease in SFT.  Of course, you’d think a service light would be the very
first indicator of a problem but that’s not always so.  At times a problem
can develop with the MAF and no service light will come on right away. 

Causes of a faulty MAF
Contamination of the MAF hot wire is the main cause of failure.  Air
itself, even after it has gone through a perfectly good-clean air filter, is
still not that clean.  Overtime a build up of gunk will slow the reaction of
the air affecting the current flow for the meter.  Cleaning does help in
some cases, but replacing with new is still the best option.
Some of the other causes of failures are:
▪ Faulty electrical connections
▪ Sensor damaged from a collision
▪ Faulty wiring
▪ Faulty PCM
All of which can be determined when diagnosing the problem.

Scanner Diagnostics
A good scanner with PID capabilities is a good method of diagnosing the problem.  Rather than go straight to code reading (which there may not be any) I prefer looking at the SFT and the MAF PID’s. 
Example vehicle:  2010 Nissan Versa  rough idle no codes
With engine warmed up, in park and A/C off (no other loads on the engine) the idle grams m/s should be 1.0 - 4.0 grams @ 2,500 rpms the value should be 4.0 - 10.0 grams m/s.  Knowing these values and watching the  PID values should clue you in on the condition of the MAF.
This same PID check can be done on any vehicle if you have the specs. but, what if you don’t?  Then what? 

MAF Diagnostics with a multimeter
Basically all MAF sensor have 4 wires.  Some have more and if so those extra wires are for the intake air temp. sensor that has been incorporated into the MAF housing.  Let’s look at the 4 main wires that make up the MAF sensor.
▪  Battery voltage 12- 14V
▪  Ground lead
▪  5V reference from PCM
▪  Signal return to PCM
Three of those wires can be checked without much guess work with your basic multi-meter.  But the real key to testing with a multi-meter is the signal return lead.  This will be easy to spot because the return voltage will be less than 5V (or better be).  If you test the other leads associated with the MAF and you see another 5V reference and a return voltage those will be for the intake air temp. sensor.  But we’ll sort those out in a minute.

MAF Snap Test
Now you’ll need one more thing to test this a bit further.  Not just a multi-meter but a multi-meter with the “min-max” feature.  This feature on the multi-meter will allow you to record a voltage reading for a brief time.  (You could use a scope for this but I’m trying to keep this simple for now.)
Hook your min/max meter leads up, one to a good ground and the other on the return lead.  Now, as quickly as you can stomp that accelerator hard to the floor.  One time, one time only.  If, you’ve got it hooked up correctly and you’re on the right return lead you should see an increase on the meter over or just above 4V.  If not, (assuming you’re on a MAF with more than 4 leads) try the other reference voltage lead.  If after all of that you don’t see an increase over 4v on the meter, that MAF is certainly bad.  (Note: There are a few older vehicles with a 3 wire MAF, the reference return lead is still the same.)
There is one exception to this quick test though.  That’s the Toyota 4cyl and some 6cyl engines.  These will barely make it to 3.8V.  So, if you’re working with a Toyota product keep that in mind.  Every other vehicle, import or domestic, it’s 4V.  This quick little test works without fail.  Try it on a known good vehicle so you can see the results as they should look then you’ll be ready to tackle that problem car.  Hope this helps.