Most likely an 02 sensor drifting. I just went through this with my 06 230,000miles. Had original 02 sensors. I had an issue with catalytic efficient too low. This was the downstream sensor. Yours would be the upstream sensor for fuel trim. You’d want to replace both together front and rear bank upstream so you don’t find yourself in the same situation down the road. I’d recommend NTK from Rock auto as this was the factory OEM. Seems like somewhere in the 200k mile range people tend to see the 02s give issues/fail

 

I have upstream o2 sensors replaced last year. I just found that ( sum of fuel trim of bank 1 high) just recently. I think maybe the fuel injectors cause this . Is it more likely than something else?

 

If no other symptoms present (running smooth, no misfires etc) I wouldn’t suspect injectors but mileage is up there. Has there been a recent valve adjustment? My valves were needing adjusted around 220k. Most other issues would seemingly affect both banks. What brand were 02? If a cheap aftermarket used then sensor could still remain a variable here

 

I had misfires last month, but after cleared the code it now not showing misfire codes (p0300-p0306).

 

I got very similar observation as you did on my 06 Exl with 340K km, bank1 combined fuel trims reading doubles the values on bank2 for couple years till now, and the colder of the weather, the bigger the values; also when van in idle, reading is way higher than in driving, highest combined 20% on b1, but in highway long driving, two banks values both dropping down to b1 1.6%, b2 0%; thought this due to vacuum leak, but i never find where is leaking; I also had random misfire codes P0300--to P0306 from time to time as well, but after erasing them, it won't act up again for long, say a year; sometime i got P0171 bank 1 lean code if van in long idle waiting....so i tried below

I changed PCV, no change, changed O2 sensors, no change, did valve adjustment, no change, so i blame this to age....and quit chasing the root causes as other than these, van drives like butter, winter highway mileage can be around 9.5-10 L/100km, ie 23 MPG, 18 Mpg in City, not bad for such age and mileage.

My routine maintainence is 8-10K km OCI with PCV cleaning, 30K Km ATF Drain and Fill with Air filter, EGR valve cleaning, top off all fluids if needed per month, incl. PS, coolant, Braking Fluid, windshield Fluid.

 

My 2006 odyssey touring did almost exactly likes you.. when I idle too long P0171 code comes after few waits... I can't find the vac leaks...

 

Thank You for your giving help. Your car very much similar to yours. There is one thing you did not do : change the fuel injectors, I think that I may try to do that once I found Hitachi fuel injectors available on Rock Auto. For now I will look into the O2 sensors testing by reviewing OBD 2 O2 numbers.

 

First, the check engine light will tell you when your perceived problem actually becomes a problem.

Second, the computer is correcting for the issue, and you say the van is running well.

Finally, you don't give numbers.