99 Avalon CEL with P0171 Code - Help!

[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 "http://www.toyotanation.com/forum/t96508.html" (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
(Note - I'm new here. Also posted this on the old thread, then realized I'd probably need a new thread. Hope this is okay.)

Quote:
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 .

[toyomoho]

The Camry V6 appears to use the same MAF sensor. Perhaps checking past Camry posts might yield info or posting your question on the Camry site would provide a larger audience.

[BadChicken]

Most of the time you just replace the MAF sensor

[DonS47]

After careful consideration and further review of other posts, I have a MAF sensor on order. Will post results tomorrow nite.

[DonS47]

Replaced MAF @ 1700 hours and went for a test drive.

Pleased to report the CEL did not come on ... for about 2 minutes.

So - back to the drawing - I mean scanning - board.

************* - 7/27 - Start Correction*******************************

Sensor & bank were misstated:

On scanning the various sensors with new ProScan 4 scanner (www.myscantool.com), found that O2 bank 1 sensor 2 (downstream) was putting out a total of 0 volts, while bank 1 sensor 1 and bank 2 sensor 1 were putting out around .6 volts.

Very suspicious!

I'm guessing my problem is with the downsteam O2 sensor.

************* End Corrections *******************************

Stay tuned.

(Inputs invited.)

Don

P.S. Way to go, Tiger!

[DonS47]

There's nothing like a scanner that tells you what is wrong:

Based on ProScan readouts of all sensors, purchaced downstream O2 sensor (Bank 1 Sensor 2) and replaced.

Problem solved. Voltage output from the new sensor varies between .05 and .88. (The old one was between 0 and .005.)

Lessons learned:
- Read the scanner manual - First.
- Pay attention to the scanner - MAF was varying with engine speed - was likely ok. But, now I have a new MAF. Upstream sensors varied with engine speed, also.
- Cal vs non-Cal emissions configuration is on sticker on firewall.
- Bank 1 and 2 Sensors 1 Cal and non-Cal sensors are different and not interchangeable. Cal sensor is Air/Fuel ratio sensor, varying current to PCM. non-Cal sensor is an O2 galvanic battery, varying voltage to PCM.
- Downstream sensor is same, Cal and non-Cal
- You really do have to remove the front passenger seat to get to the connector for the downstream sensor. You also have to remove the rear portion of the console. (Shift lever trim pries up, to remove.)

Don