This works for both 2.4 and 3.0. This guide is based on the 3 litire model, but you can do the same process with the 2.4. You just need to locate your MAF sensor beforehand. This will be situated after the air Filter box and the engine.
MAF Sensor on the Toyota Alphard - 3.0 V6
What is a MAF sensor?
Your mass air flow sensor or “MAF” detects the volume of air as it enters your engine by heating up a thin wire or resistor. The computer in your car then measures electrical resistance through the MAF as the air flowing through the sensor cools its components down. With a firm understanding of how much air is flowing into your engine, the computer can then direct fuel injectors to add more or less gasoline to the air/fuel mixture.
First off you are going to need a can of Isopropyl Alcohol (excuse the state of my old can 
) Also be weary of buying MAF sensor cleaner. It's the same stuff and it's twice the cost. Go to your local maplin or ebay and purchase a can for around £4
You also need a stubby screwdriver. I lost mine so using a short screwdriver.
Unscrew the top housing of the sensor and gently remove.
This will expose the red diode (does not look red at the moment as it's dirty) Spray this diode with Isopropyl liberally.
Now for the dodgy bit. Some people recommend not to touch the sensor, however, i have always given mine a very light rub with a cotton wool bud. So be careful here.
The dirt is now off the diode.
Now look inside the may sensor assembly and you will see two small blades. Give then a good squirt.
Shake off residual alcohol and wait for the sensor to dry. Takes about 5 mins.
Refit to car.
If you had a Check engine light due to the MAF sensor then this is a good time to disconnect the negative terminal on the battery for 15 mins to reset the ECU and clear any codes. I have seen the car also go in to limp home mode by simply having a dirty sensor.
The dealer hates people knowing how to clean these as most of the time all they need is a simply clean while the dealer wants to help himself to £200 from you for simply replacing a dirty sensor. All Done for just £4 and 15 mins you could say a whole bunch of notes.
Notice: you are responsible for anything you do to your vehicle from this guide. We cannot be held responsible if you break something. Please do so at your own risk.