MTNTOY

Would a clogged Cat cause a bank one too lean code of P0171?
Also, would replacing my Cat. with a straight pipe cause problems for my O2 sensors?

lee

no, thats a MAF.

SEARCH https://www.yotatech.com/forums/show...ighlight=p0171

MTNTOY

Just replaced my MAF and still getting p0171

MTL_4runner

Was it a new or used part?

If it was new and you still get the code you should follow the FSM for proper diagnosis of the problem.
Most of the time it is the MAF with that code but a rare case proves otherwise.

MTNTOY

Used MAF from e-bay...that may be my problem

MTL_4runner

Unfortunately....
Go try a junkyard and see if you can get a good one that clears the code.
I wouldn't put it past some jerk selling a lemon on ebay (if it is get your money back)

MTNTOY

Before buying the MAF I made sure I would be able to get my money back...Is there any other way of determining if the MAF is indeed bad or something else causing my lean code?

MTL_4runner

You can test them but I am not sure how good the results will be.
This is for GM sensors but Toyota uses the same technology so testing procedure is also similar:

TESTING
MAF sensors can be tested either on or off the vehicle in a variety of ways. You can use a MAF Sensor Tester and tachometer to check the sensor’s response. If testing on the vehicle, unplug the wiring harness connector from the sensor and connect the tester and tachometer. Start the engine and watch the readings. They should change as the throttle is opened and closed. No change would indicate a bad sensor. The same hookup can be used to test the MAF sensor off the vehicle. When you blow through the sensor, the readings should change if the sensor is detecting the change in air flow.

Another check is to read the sensor’s voltage or frequency output on the vehicle. With Bosch hot-wire MAF sensors, the output voltage can be read directly with a digital voltmeter by back probing the brown-and white output wire to terminal B6 on the PCM. The voltage reading should be around 2.5 volts. If out of range, or if the sensor’s voltage output fails to increase when the throttle is opened with the engine running, the sensor may be defective. Check the orange and black feed wire for 12 volts, and the black wire for a good ground. Power to the MAF sensor is provided through a pair of relays one for power, one for the burn-off cleaning cycle), so check the relays too, if the MAF sensor appears to be dead or sluggish. If the sensor works but is slow to respond to changes in air flow, the problem may be a contaminated sensing element caused by a failure in the self-cleaning circuit or relay. With GM Delco MAF sensors, attach a digital voltmeter to the appropriate MAF sensor output terminal. With the engine idling, the sensor should output a steady 2.5 volts. Tap lightly on the sensor and note the meter reading. A good sensor should show no change. If the meter reading jumps and/or the engine momentarily misfires, the sensor is bad and needs to be replaced. You can also check for heat-related problems by heating the sensor with a hair dryer and repeating the test. This same test can also be done using a meter that reads frequency. The older AC Delco MAF sensors (like a 2.8L V6) should show a steady reading of 32 Hz at idle to about 75 Hz at 3,500 rpm. The later model units (like those on a 3800 V6 with the Hitachi MAF sensor) should read about 2.9 kHz at idle and 5.0 kHz at 3,500 rpm. If tapping on the MAF sensor produces a sudden change in the frequency signal, it’s time for a new sensor.

On GM hot-film MAFs, you can also use a scan tool to read the sensor’s output in “grams per second” (gps), which corresponds to frequency. The reading should go from 4 to 8 gps at idle up to 100 to 240 gps at wide-open throttle. Like throttle position sensors, there should be smooth linear transition in sensor output as engine speed and load change. If the readings jump all over the place, the computer won’t be able to deliver the right air/fuel mixture and driveability and emissions will suffer. So you should also check the sensor’s output at various speeds to see that its output changes appropriately.

Another way to observe the sensor’s output is to look at its waveform on an oscilloscope. The waveform should be square and show a gradual increase in frequency as engine speed and load increase. Any skips or sudden jumps or excessive noise in the pattern would tell you the sensor needs to be replaced.

Best way to see output is with an OBD II reader that will display the output of the MAF while in operation.
The true way to test is detailed in the FSM.

MTNTOY

Thanks for all your help with this. I really appreciate it.