I have an old Volvo s40. Because I had gotten a P0420 code and because of it, an OBD scan tool, I noticed that my fuel trims were very high. Or they could be the result of the age of the car (17 years old and over 200k miles). My Ltft is over 10% and was even over 13, but I bought oxicat cat cleaner additive to see if it will help. And it actually did I think. Over the course of a week and a half, it dropped to 10% and today briefly to under 10,but after a brief rest, it went back to over 10%.
Now, both my upstream and downstream sensors oscillate between the high of 1.275/1.28v and any value in between. I get lots of ltft of up to 10% even in idle. Here are screenshots of the graphs of my obd scan results while driving https://imgur.com/a/zE2cMgy
Is this normal behavior of preheated o2 sensors, presumably narrowband?