The 5 Second Air Fuel Ratio Reset Procedure - Second Generation Nissan Xterra Forums (2005+)
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post #1 of 24 (permalink) Old 04-11-2009, 04:07 PM Thread Starter
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The 5 Second Air Fuel Ratio Reset Procedure

I was looking in the service manual for information about the mass air flow sensor when I stumbled upon a handy procedure.

Here's how to reset the "air fuel ratio self-learning control value" (discussed along with "ECU relearning" as "reset the ECU" or "ECU reset" in other threads) without disconnecting the battery, waiting for some unknown number of minutes or hours, and losing other settings like the radio station presets in the process.

This procedure takes 5 seconds. Well, not exactly... It takes a bit longer than that, but the basic trick is to disconnect the MAF for at least 5 seconds.

I would appreciate a second opinion from somebody, please, in case I'm sadly mistaken!

Here are the relevant excerpts from the 2006 service manual. I haven't checked the manuals for other years yet, but I would ASSUME (as in maybe YOU shouldn't) that the procedure is the same. [update: Checked 2005 - identical procedure in "DTC P1273, P1283 A/F SENSOR 1" on page EC-510]

Note that steps 6 through 8 are easy if you have a ScanGuage. If you don't have one, you can perform the "HOW TO ERASE EMISSION-RELATED DIAGNOSTIC INFORMATION" procedure manually, or you can just disconnect the battery and wait...

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Extracts from the Engine Control (EC) section of the 2006 Xterra service manual
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[from page EC-25] MIXTURE RATIO SELF-LEARNING CONTROL

The mixture ratio feedback control system monitors the mixture ratio signal transmitted from air fuel ratio (A/F) sensor 1. This feedback signal is then sent to the ECM. The ECM controls the basic mixture ratio as close to the theoretical mixture ratio as possible. However, the basic mixture ratio is not necessarily controlled as originally designed. Both manufacturing differences (i.e., mass air flow sensor hot wire) and characteristic changes during operation (i.e., fuel injector clogging) directly affect mixture ratio. Accordingly, the difference between the basic and theoretical mixture ratios is monitored in this system. This is then computed in terms of “injection pulse duration” to automatically compensate for the difference between the two ratios.

“Fuel trim” refers to the feedback compensation value compared against the basic injection duration. Fuel trim includes short term fuel trim and long term fuel trim. “Short term fuel trim” is the short-term fuel compensation used to maintain the mixture ratio at its theoretical value. The signal from air fuel ratio (A/F) sensor 1 indicates whether the mixture ratio is RICH or LEAN compared to the theoretical value. The signal then triggers a reduction in fuel volume if the mixture ratio is rich, and an increase in fuel volume if it is lean.

“Long term fuel trim” is overall fuel compensation carried out long-term to compensate for continual deviation of the short term fuel trim from the central value. Such deviation will occur due to individual engine differences,wear over time and changes in the usage environment.

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[from page EC-116]

[work item] SELF-LEARNING CONT
[condition] THE COEFFICIENT OF SELF-LEARNING CONTROL MIXTURE RATIO RETURNS TO THE ORIGINAL COEFFICIENT.
[usage] When clearing the coefficient of self-learning control value

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[found in section] DTC P0133, P0153 A/F SENSOR 1

[from page EC-265] 5. CLEAR THE SELF-LEARNING DATA [the manual SELF-LEARNING CONT procedure]

1. Start engine and warm it up to normal operating temperature.
2. Turn ignition switch OFF.
3. Disconnect mass air flow sensor harness connector.
4. Restart engine and let it idle for at least 5 seconds.
5. Stop engine and reconnect mass air flow sensor harness connector.
6. Make sure DTC P0102 is displayed.
7. Erase the DTC memory. Refer to EC-61, "HOW TO ERASE EMISSION-RELATED DIAGNOSTIC INFORMATION" .
8. Make sure DTC P0000 is displayed.
9. Run engine for at least 10 minutes at idle speed.
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post #2 of 24 (permalink) Old 04-14-2009, 03:03 PM
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Truck SHOULDN'T run without the MAS hooked up.

If it does work let us know!
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post #3 of 24 (permalink) Old 04-14-2009, 03:20 PM
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Vehicles will run with almost every sensor disconnected. TPS, Crank, O2, MAF etc...

It just won't run without the ECU.
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post #4 of 24 (permalink) Old 04-14-2009, 03:47 PM
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and the benefits of doing this would be........??
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post #5 of 24 (permalink) Old 04-14-2009, 04:08 PM
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is this the trick to up the mileage?
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post #6 of 24 (permalink) Old 04-16-2009, 01:12 PM
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Sounds like it resets/relearns the optimum Air/Fuel mixture for the engine? So I guess that implies that the A/F mixture is fixed after you break in your truck?

Might be interesting to try if you added a CAI. If the air/fuel ratio is fixed, then does that mean that CAI's make your engine run leaner?
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post #7 of 24 (permalink) Old 04-16-2009, 01:54 PM Thread Starter
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This procedure simply resets the short and long term fuel trim maps which are used to modify fuel injector pulse widths.

Fuel trim maps (both short and long) are used to lean out or richen the fuel mixture in an attempt to maintain the optimal 14.7:1 air to fuel ratio and these trim cell values are "learned" over time. STFT (short term fuel trim) values are adjusted almost instantly. LTFT values are a longer term average, but are still subject to adjustment at any time.

If you add a CAI, you shouldn't run lean for more than a few seconds, because STFT values should compensate.

From what I've read, some people believe that resetting trim values is worthwhile after making certain intake, exhaust, or fuel delivery modifications. Other people believe that it's a waste of time and may, in the short term, be deleterious. I'm not sure WHAT to believe, but I'm inclined to think that it's not worth doing.
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post #8 of 24 (permalink) Old 02-24-2011, 11:17 AM
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Quote:
Originally Posted by Shiny View Post
This procedure simply resets the short and long term fuel trim maps which are used to modify fuel injector pulse widths.

Fuel trim maps (both short and long) are used to lean out or richen the fuel mixture in an attempt to maintain the optimal 14.7:1 air to fuel ratio and these trim cell values are "learned" over time. STFT (short term fuel trim) values are adjusted almost instantly. LTFT values are a longer term average, but are still subject to adjustment at any time.

If you add a CAI, you shouldn't run lean for more than a few seconds, because STFT values should compensate.

From what I've read, some people believe that resetting trim values is worthwhile after making certain intake, exhaust, or fuel delivery modifications. Other people believe that it's a waste of time and may, in the short term, be deleterious. I'm not sure WHAT to believe, but I'm inclined to think that it's not worth doing.
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post #9 of 24 (permalink) Old 02-24-2011, 11:21 AM
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I just did the reset this morning. Ill post my findings when I get some mileage. I was getting 10-13 mpg roughly city driving. All stock 06 S model with 4 wheel drive and almost 44,000 miles on the X. Driving conditions were conservative not going above 2000 rpm's very often.
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post #10 of 24 (permalink) Old 02-26-2011, 04:03 PM
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Let us know the results.Right now I`m running code PO 171 which is system running too lean[bank 1].Anybody else have any input on this?
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