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The numbers were positive. You understood correctly.

I was thinking about that too - seems odd that the ratio was what it was if a dirty filter was the issue.

I did have to reset the check engine light. While I had the air intake off, I turned the ingition on (not the engine) and put a brick on the gas pedal just to open up the throttle to clean it. After I put everything back together and took it for a drive, I noticed the check engine light on. I figured it just took offense to not having those couple of sensors connected when I had turned it on. So when I got back, I cleared those codes, and that's when I checked the fuel trims again.

So I don't know if clearing the codes had anything to do with it.
 

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Discussion Starter #22
Yes, if you clear the codes you also clear the long and short term fuel trims and they reset to zero. They will work their way back up if the problem is still there.
 

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Ah, ok. That makes sense. I'll make sure to tell my wife that I didn't spend $300 in vain :)

I did notice it was idling slightly higher. By slightly I mean in the 900's rather than the 700's as before. Once I get the O2 sensors in I'll make sure to reset the PCM and go through the idle relearn.
 

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Discussion Starter #24
Yeah, 900 is too high. You need a bidirectional scan tool to do the PCM reset and idle relearn. If you have a VCMuzzler you'll want to remove it before doing the reset and relearn as during the idle relearn process the ECT1 temp needs to reach 194F, which may not happen with the Muzzler installed. Then once you've done that you can reinstall your Muzzler. This is where a variable resistor type Muzzler comes in handy...dial it down to zero, do the relearn, and then dial it back up to the proper setting again.
 

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You sure you need a bidirectional scanner?

I'm just looking at the manual now. It says this about the PCM reset (page 11-4):
Sean Tool Clear Command
If you are using a generic scan tool to clear commands,
be aware that there is only one setting for clearing the
PCM, and it clears all commands at the same time (CKP
pattern learn, idle learn, readiness codes, freeze data,
on-board snapshot, and DTCs). After you clear all
commands, you then need to do these procedures, in
this order: PCM Idle learn procedure (see page 11-410);
CKP pattern learn procedure;
Test-drive to set readiness codes to complete (see page
11- 100).
What I understand from that is that clearing the codes means I just reset the PCM.

Then the idle relearn instructions on page 11-410 don't seem to require a scanner at all either:
Procedure
1. Make sure all electrical items (A/C, audio, rear
window defogger, lights, etc.) are off.
2. Reset the PCM with the HDS.
3. Turn the ignition switch to OSM (II), and wait
2 seconds.
4. Start the engine. Hold the engine speed at
3,000 rpm without load (in P or N) until the radiator
fan comes on, or until the engine coolant
temperature reaches 134 °F (90 °C).
5. Let the engine idle for about 5 minutes w ith the
throttle fully closed.
NOTE: If the radiator fan comes on, do not include
its running t ime in the 5 minutes.
But good point on the muzzler. I wouldn't have thought of that.
 

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Discussion Starter #26
100% sure. I've been this road before. When they mention "generic scan tool" they're referring to something that's not the Honda Diagnostic System (HDS.) Resetting the PCM is much different than clearing the codes with an OBDII scanner.

A bidirectional scan tool used to reset the PCM will reset everything and you have to do the CKP relearn (doesn't require a scan tool) and the idle relearn (does require a bidirectional scan tool.)
 

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Now I know you have a ton more experience here than I do, so forgive me :) But I understand that when the manual says "generic scan tool", is means a non-HDS (and non-bidirectional) scan tool. But it also seems to suggest that those generic scanners can't pick and choose what it clears - it can only clear everything. So if the DTC's get cleared, everything, including the PCM and idle learn, is cleared.

Are you saying otherwise based on experience? Or some other documentation?

I still haven't pulled the trigger on that Foxwell NT510 that you suggested. But I think I will after we get moved.
 

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Discussion Starter #28 (Edited)
"Generic scan tool" does not necessarily mean non-bidirectional. Remember, the manual is written for professional shops, including the dealerships, that often use the SnapOn Verus and other full functioning scan tools. If you clear the codes with your current OBDII code reader it will not clear the idle learn, ckp, and pcm. It will only clear codes. On the bidirectional scan tools that CAN clear that data it often clears all of it when you are in the OEM data functions with the generic scan tool.

I can guarantee that your standard OBDII scan tool will NOT allow you to relearn the idle. I have one and it did not allow me to do an idle relearn. I learned this the hard way with my Autel MD802 which actually reads all the OEM data for almost all vehicles but has no bidirectional controls. I was only able to do the idle relearn after using the Foxwell NT510 to either do a PCM reset or clear the Throttle Position data...the tool has both functions. My little OBDII scanner and my Autel MD802 do not.
 

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Discussion Starter #29
You've actually proven it to yourself since you reset your codes with your scanner and your idle is still in the 900 RPM range. When you properly reset the PCM or clear the throttle position data the idle will drop down around 600-650 and the idle relearn would then take place.
 

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Discussion Starter #31
It's a great little scan tool, for sure.
 

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Discussion Starter #33
The PCM should still be reset after changing out the O2's. You might get the vehicle to relearn the idle after 2-4 weeks but often a high idle will cause the vehicle to not slow down when you let off the gas. In my case it would cruise down the road at 30-40mph without my foot on the gas. Not real safe.
 

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Discussion Starter #35
That is a good deal. You did get the one with Honda installed, correct? They come with one manufacturer either preinstalled or no manufacturer and you can download one for free. Additional manufacturers are $60 each, up to 5, if available.
 

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Discussion Starter #37
I think you'll be very pleasantly surprised by the features that thing packs for the money.
 

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I'm sorry to dig up an old but excellent thread. Thanks for the write-up, John!
My 2007 EX-L has 120k on it the LTFT is 3.9% which is higher than I want. I put off the idea of replacing the 4 sensors because they are SOOOOO EXPENSIVE from Honda. As an example, the bank2 downstream(Honda part# 35632-RJA-004) is $553+tax. It's a narrow band old style O2 sensor, why the h* Honda want that much for it? Interestingly, the downstream sensor on bank1 is $204..

Is the NTK sensors($40 something) the same OE sensor or it's "OE compatible" sensors? I'm the kind of guy to use OE parts all the way but if the price difference is 10x, that's another story.

So, I have been working on this problem for a while. My 08 Touring has been running lean for the last few months and not getting very good gas mileage either. It hasn't set any codes...I only put the codes in the title for search purposes but the PCM has been adding fuel to correct for what it sees as a lean condition.

An oil change or two ago I connected my scan tool as I routinely do at oil changes. This particular time I noticed that my fuel trims were running around +10% on both short term and long term. If you add them together that's a total fuel trim of +20%. That means the PCM is adding 20% more fuel to compensate for a lean condition. When running the RPM's up to 2K or 3K the fuel trims stayed about the same which I figured ruled out a vacuum leak, however, I searched for one anyway but did not find anything. Scan tool indicated the EGR was commanded closed and indicating closed so I didn't suspect an EGR problem.

Since both banks were about the same I figured possibly a mass air flow sensor. I removed it and cleaned it to no avail. With nearly 170K on the clock I started thinking O2 sensors. That's where things get a little tricky. On the 05 and up Honda used air fuel ratio (wideband) sensors rather than standard O2 sensors. These can be much trickier to diagnose because they don't cycle rich-lean-rich-lean like normal sensors. So, with the mileage I went ahead and replaced both upstream A/F ratio sensors. That seemed to help and the fuel trims came down to under 5% on both short and long term. However, after a few hundred miles I checked again and my fuel trims were back up to 10-20% total between ST and LT again. WTH?

After another month or two of checking scan data and further research I figured it had to be either MAF or downstream O2's. I was able to drive the sensors rich and lean when desired and they appeared to work correctly. However, I decided to change the downstream O2's anyway. These are standard narrow band O2's and, in most cases, are usually just for checking catalyst efficiency and comparing to the front O2 sensor. However, it turns out that on A/F ratio equipped Hondas the PCM does use the downstream sensors for fuel trim adjustment. What I didn't know was to what extent. I mean, if Honda puts in the fancy wideband, much more accurate, A/F ratio sensors on the front how much control could the rears have?

So, I went ahead and changed the rear O2's last week. After some monitoring I saw the fuel trims come down a bit to around 10-13% total, so there was some improvement. A bit of driving around town and they were down to around 8-10% total. With a good repair I'm used to seeing an opposite short term correction that will equal the long term correction. For example, if the long terms are at +10% and you repair the problem, you'll see the short term tick down to -10% which will eventually push the long term fuel trims back to zero. If you simply add the short and long terms and they equal zero then you usually have a good repair. I wasn't seeing that. At that point, I figured I must have a MAF problem so I went ahead and ordered a new MAF.

The new MAF showed up today. However, before just throwing it in I figured I'd check the fuel trims so I could compare them to the post MAF installation fuel trims. To my surprise, my long term fuel trims were already at 0 and the short terms were around -1 to +2, nearly perfect, or even a little rich of normal with the PCM subtracting a little fuel. I can only surmise that it takes a bit longer than normal for the computer to adjust the fuel trims after a component change like that. I didn't reset codes (since it didn't have any codes) or disconnect the battery to reset all that...I just watched to see if the PCM made any corrections to confirm the fix. Apparently, I didn't watch long enough because the PCM finally brought things back to normal.

The reason for this post is that NONE of this is in the Honda FSM and there is VERY little about Honda lean codes on the Internet. If you get a lean or rich code (or even if you just have a lean/rich condition but no code like I did) all Honda says to do is check fuel pressure (not easy to do,) check the MAF sensor g/s at 2500 RPM, inspect valve clearances and then replace injectors if none of that fixes it. It says that if the LT fuel trim stays within -22 - +25 then there is no problem. The lean/rich codes set at -22 or +25% on LT fuel trim. The problem is that if the O2's are old and not working efficiently then you can have +22% fuel trim, crappy gas mileage, no CEL and it could eventually take out your catalytic converter. I often wonder if this is why there are so many surprise P0420/P0430 codes on Hondas with no prior warning.

So, my take on this is that if you have high mileage on your J35 engine it's probably a good idea to replace ALL FOUR oxygen sensors as a maintenance item. If you have a scan tool you can keep an eye on them like I did. If not, then I'd recommend just changing them by no later than 150K. I bought my OEM NTK sensors on RockAuto and found some service life info from NTK there:

https://www.rockauto.com/en/moreinfo.php?pk=2258828&cc=1441873&jsn=365

Essentially, it says for modern 4 or 5 wire sensors the service life is about 100K miles. So, if you're over 100K miles and scan data is showing much over 10% total fuel trim adjustment, I'd consider replacing ALL FOUR sensors. Don't just replace the two upstreams. The downstreams have a lot to do with controlling fuel.

Hope this helps someone...

John

Another document from NGK on sensor life expectancy:

https://www.ngkplugpro.ca/content/contentfiles/pdf/NTKO2-0308-1LifeExpectancyofOxygenSensors.pdf
 

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Discussion Starter #39
I'm sorry to dig up an old but excellent thread. Thanks for the write-up, John!
My 2007 EX-L has 120k on it the LTFT is 3.9% which is higher than I want. I put off the idea of replacing the 4 sensors because they are SOOOOO EXPENSIVE from Honda. As an example, the bank2 downstream(Honda part# 35632-RJA-004) is $553+tax. It's a narrow band old style O2 sensor, why the h* Honda want that much for it? Interestingly, the downstream sensor on bank1 is $204..

Is the NTK sensors($40 something) the same OE sensor or it's "OE compatible" sensors? I'm the kind of guy to use OE parts all the way but if the price difference is 10x, that's another story.
First, 3.9% is perfectly normal. +/- 5% is considered normal and I usually don't do anything until it goes above 10%. I wouldn't do anything at all.

Second, NTK is the OE sensor. You can buy them on RockAuto and they are the exact same sensor you'd buy from Honda but without the Honda name on it.
 
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