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Discussion Starter · #1 · (Edited)
My sister's '01 L was showing a check engine light with four emissions codes, including P0174. The other codes will be covered in a different post as they were related to the catalyst/oxygen sensors. This post shows a common fix to clear the P0174 by simply cleaning the Mass Air Flow sensor (MAF).

This is an easy fix, which is described well in many videos on youtube, such as this one:


To remove the sensor on the 4.2, you can either remove the plastic cowl piece above it to gain enough room to remove the two screws that hold the MAF sensor in the air intake housing, or you can remove the air intake housing in which it is seated, and then remove the sensor.

I chose not to remove the cowl because I didn't have any of the little plastic clips that hold it in place and that seem to always break when you mess with them. Also, I planned on cleaning the throttle body while I was in there, so I had to remove the air intake anyway. It is easy to remove the air intake.


SUPPLIES and TOOLS

10-mm socket (or wrench)
CRC Mass Air Flow Sensor Cleaner or an equivalent, high-quality MAF sensor cleaner:
Do not use throttle body cleaner, or brake cleaner, or carb cleaner or anything like that. Use only MAF sensor cleaner. The CRC MAF cleaner is available at auto parts stores and even Home Depot: expect to use about 1/3 to 1/2 a can per cleaning.​
Small head Phillips screwdriver
Pliers or channel-locks


STEPS:

0.) Disconnect the negative terminal on the battery.

1.) Locate the MAF sensor on the top of the air intake, facing slightly back. Unplug the electrical connection to the MAF sensor.




2.) Unclip the two clamps on the passenger's side of the air intake housing, and pull back the front of the intake to expose the air filter. Remove the air filter.


3.) Remove the two 10-mm bolts which fasten the air intake housing to the valve cover.


4.) Disconnect the vacuum hose from the air intake by pinching the small clamp and pulling the hose back.




5.) Disconnect the PCV hose that runs from the valve cover to the air intake housing by pulling the PCV hose out of the air intake hose: this is just a fitted piece with a hard plastic end and there is no clip.




6.) Remove the bracket that sits in a rubber slot feature that is part of the air intake hose. This bracket is just pushed in place into a slot. Remove the bracket from the hose by pulling up on it (note -- it is very snug, but you just have to keep working it till it comes out).




7.) Open the clamp that connects the hose to the throttle body. This clamp should have a little plastic clip that will lock the clamp OPEN once you pry it open, but your little plastic clip may be missing or broken. Use a channel locks or large pliers to open the clamp.

If your plastic clip is broken or missing, you will have to hold the clamp open while you remove the hose.
When it comes time to re-install it, you can use a zip-tie to hold the clamp open so you can place it over the throttle body opening, then just snip the zip-tie to release the clamp once you have it re-installed).​


8.) Now you can remove the air intake housing by simply pulling it out. There is a plastic alignment boss on the underside of the housing up by the valve cover that you might have to wiggle a bit to free it, but it will come out.


9.) Now that you have the intake out, you can easily access the MAF sensor. To remove the MAF sensor, remove the two small Phillips screws and pull the sensor out:

Handle the sensor carefully, and don't drop it. You will see on the outside a larger feature which looks like a matchstick: this is the air velocity component of the sensor. Turn the sensor so you can look inside and you will see two very small amenometers. Now check out this great post with excellent macro pictures to see the parts that you will be carefully cleaning: https://howtune.com/articles/161-clean-the-maf-sensor-on-a-lotus-elise


10.) Use CRC Mass Air Flow Sensor Cleaner to clean all of these components, as well as the electrical connector. Do not use Throttle Body Cleaner, or Carb Cleaner -- use only a cleaner specifically designed for MAF sensors. Let the sensor dry completely before re-installing it (it should be dry in about an hour).

When cleaning it, be sure not to touch or contact the sensor components, especially the anemometers. Spray them well using the straw on the CRC MAF Cleaner, but don't touch them. Expect to use about half the can of cleaner per cleaning. It works well to spray it down, then let it dry for about 20 minutes, then spray it again to knock free more dirt. The anemometers should be very clean by the time you're done.​


11.) Re-install the components in reverse order.


12.) The code will likely clear due to having had the battery disconnected for a little while. After reconnecting the battery, the engine might seem to idle a little low -- go for a drive for about 10 minutes to let the ECU re-learn the conditions. You should notice an improvement in the idle after the computer re-learns, and hopefully the code will be permanently cleared.
 

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Discussion Starter · #2 ·
Clarification regarding #5

Clarification -- Regarding #5, that is the ventilation hose from the valve cover to the air intake (not the regulated ventilation hose that has the actual PCV valve on one end). You don't have to remove anything with the PCV valve or its hose to remove the air intake.

Please refer to this picture for clarification:

 

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So your profile says you have a 2001 Rav4 with the I-4 engine, so yours, as far as I know should be the exact same as mine, unless it has been modified. Some vehicles use a intake manifold absolute pressure sensor instead of a mass airflow sensor.
If you could post a photo of your engine bay that would help alot.
 
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