VCS and Check Engine Indicators

[DonS47]

I've got a 99 Avalon XLS, 6 cyl, with 183k miles. No personal history with the car - just bought it. CEL came on, while driving it home.

Read all the posts on the thread "Check Engine light - Vehicle Speed Sensor " (see quote, below).

I used an OBD II scanner and found only a P0171 code - no others. The Haynes manual lists six potential problem areas but no troubleshooting sequence, so that's when I searched the forum. Here's where I started:

- Loose air intake hose or duct - Checked this out. All ducts and filter cover look tight. (Just put in a new filter, after cleaning out the field mouse nest.)

- Incorrect fuel line pressure - Don't have tools to check.

- Obstruction in injector - Don't know how to check this, yet.

- Heated O2 sensor problem - There were no O2-related codes in memory, so I took this to mean the problem is not any of the O2 sensors. Is this a correct assumption?

- MAP or MAF sensor malfuntion -
Didn't do anything with MAP sensor.
Took the MAF sensor out and tried cleaning the wires with a q-tip and brake cleaner. My sensor has a cover over the wires, and it was difficult to reach the wires or to know if they were dirty and got clean. Put the unit back in, cleared the code and started the engine. Scanner said there are no codes but there is a P0171 pending. (I took this to mean I didn't fix the problem.)
FROM THE PICTURES IN THE THREAD, it looks like the cover over the wires comes off, but I'm afraid to pry a $200-$300 plastic part. I've got the MAF sensor on my computer table. Per Haynes, I checked the resistance on the cold sensor between terminals 4 and 5 and got about 1.45k ohms.
Can anyone offer any info on further cleaning or testing the part, taking it apart, etc?
(I don't know how to insert the picture into this post. )

- ECT Sensor Malfunction - Didn't go there yet, either.

Besides working on the MAF, what should I be doing next?

Thanks,
Don

Originally Posted by csaxon

<snip>

Most of the time when PO171 is stored the wires of the mass air flow sensor need cleaning.
Use an electrical contact spray cleaner or brake cleaner with a Q-tip to clean the wires. Be careful not to damage them

If you have a scan tool check the fuel trim data before and after cleaning the MAF sensor. That way you'll now if the repair was effective. Without checking the data you'll have to wait and see if CEL returns. That may happen right away or it may take a number of drive cycles if the sensor is borderline.

If cleaning makes a difference but fuel trim is still correcting for a lean condition chances are you'll have to replace the sensor.


Try that and erase the code .

[utah33]

Don,

My MAF sensor did not have a cover over the sensor wires, (2000 Avalon)

You remove the two screws, unplug the sensor, and pull it out of the air cleaner, flip it over and look up into the bottom of the sensor. (The part of the sensor that extends in to the air flow path)
You may have to shine a flashlight up the little tube in order to view the two sensor wires. I didn't see them either at first, when I performed the Haynes manual check on the sensor.

Look at csaxon's photo on 11/22/05 again.

Hope you fix it.

[DonS47]

Thanks for the feedback. Overall outcome:

Replaced the MAF - no change. Should have read the manual for my ProScan software first. You could see the MAF responding to engine rpm.

On closer attention to the ProScan scanning matrix, I saw that the downstream O2 sensor was 'dead'. It was putting out between 0 and .005 v. Should have been in .5V range. Upstream sensors (Calif Air/Fuel Ratio sensors) were in normal operating range and varying with RPMs. (Massachusetts requires Calif emissions standards.)

Replaced downstream O2 sensor, and the car runs like new - at least what I presume it ran like when new. (Calif and non-Calif downstream sensors are the same.)

Note Calif A/F sensors ~ $170 vs non-Calif O2 sensors ~ $80. Lucked out there!

Also replaced left rear hub ass'y. Took care of hum in rear end.

All's well that ends well.

Don

[SwampFox]

csaxon, is right on with this diagnosis in my situation. I had an Avalon with the VSC check engine light on @ 38,000 miles. I pulled out the MAF sensor and cleaned the sensor wires thoroughly with brake cleaner aerosol spray and reassembled. I then disconnected and reconnected the battery ground about 20 seconds to clear the VSC warning light. THANKS!

Originally Posted by csaxon

Have you had work done recently? Did you replace an air filter? Look for loose or disconnected air/vacuum hoses and check your air filter box is sealed around the edges. If they're ok then: Badchicken is correct, but sometimes a simple cleaning can work wonders and save money.

Most of the time when PO171 is stored the wires of the mass air flow sensor need cleaning.
Use an electrical contact spray cleaner or brake cleaner with a Q-tip to clean the wires. Be careful not to damage them

If you have a scan tool check the fuel trim data before and after cleaning the MAF sensor. That way you'll now if the repair was effective. Without checking the data you'll have to wait and see if CEL returns. That may happen right away or it may take a number of drive cycles if the sensor is borderline.

If cleaning makes a difference but fuel trim is still correcting for a lean condition chances are you'll have to replace the sensor.


Try that and erase the code .

[ccpyue]

Originally Posted by utah33

Cofirmed Fix, Thanks to csaxon's advice.

Cleaning the MAF Sensor as described by csaxon fixed my problem. Hallelujah!!!

Thank You csaxon and everyone else that had input, it was all helpful.

The photo were great and very helpful. I had taken off this sensor before and did some ohm checks, but didn't realize that the sensor wires were up inside that tube (see photos)

After reading csaxons advice on 3-6-06, I took off the MAF Sensor and took it to work.

I am a electronic technician and had access to Q-tips and Electronic Cleaner and did just as csaxon said.

As of today, the VSC and Check Engine Light has not came on again. I wanted to give it a long run to confirm the fix. By George I believe its fixed.

Thanks again and I hope this thread helps someone else. Thats what its all about isn't it.

My '00 Avalon has the same problem. Could you tell me where is the MAF sensor locate, and how to reach it to take it off for cleaning? Thank you.

[csaxon]

[ccpyue]

Originally Posted by csaxon

Thank you very much.

[blacklove0607]

Thanks so much for this information!!! My VSC light came on after I installed a new air filter, a hose became disconnected. As a female I always cringe when I walk into an auto mechanic shop at the thought of what it's going to cost. At least I walk in with additional information when I received the diagnostic on my 02' Highlander. Oh yeah, from reading the threads, I decided to have the timing belt work done as well since I was 30,000 miles past the recommended maintenance.

Thanks again!

Blacklove

[Mytman]

I'm having problem with P0107 Manifold Absolut pressure sensor. Where is it anh how I fix it?

Thanks
Mytman

[marstil]

My '07 had the light come on after I had changed the gas cap to a locking gas cap; (don't ask...I was on the net late one night and thought a locking cap would be a good idea. Didn't recall the car has a locking gas door until after the cap had arrived). I had read somewhere that not tightening the cap could cause the check engine light to come on. I put the old cap on, tightened it 'til it clicks a few times. About two days later the lights went off and they've stayed off for several thousand miles, now. The check engine light problem could be just that simple. Try it before spending any extra dough on sensors or catalytic converters. Be sure to drive it a few days afterwards before expecting the light to go off. Mine took two days.