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
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