DanForest

My 2010 Forester also has a recurring P0172 code. Got the car from my father who tried its best to get rid of it for three years, but never could fix it. I thougth I succeeded this winter after I changed the oversized exhaust lines and aftermarket silencer the first owner had put on, but after a month, the P0172 Bank 1 too rich code came back. My father had changed the upstream O2 sensor many time and both the upstream and downstream sensor at the same time once (there are reports of code appearing after changing just the front sensor), but still the same problem came back soon each time. He also had the mass air flow sensor replaced once. My trusted mechanics's son also had a 2010 Forester and had the same code over and over. They rebuilt the entire engine (gasket, O2 sensors and other things he didn't mentionned), and the code came back a year after. He's educated guess is that Subaru has programmed the ECU for way too narrow specs for the O2 sensors readings. In other words the margins of tolerance for the O2 sensors value is too small before it triggers the code. In that case, many unperfect things can trigger the code, including vacuum leaks, mass air flow sensor getting older, wiring imperfections, air filter clogged, fuel filter getting old, in addition to actual O2 sensor defects. About the fuel filter, it is located inside the gas tank in the 2010 models, so changing it is not an option. Sometimes last winter (I live in Canada), the dash lights on my forester (check engine + loss of cruise control + loss of traction control) would come on and off several times within the same week. Usually dry cold whether would help get rid of it while slushy humid temperatures would trigger the code. Correct me if I am wrong, but this is impossible to get from a bad O2 sensor as these do not come back when they are toasted. I was hoping to get more info about this on the web and on this forum, but it appears I already know as much as there is to know. Please throw in all additonal info if I am wrong. Since this is a very common problem on these cars, I am surprised nobody can say more about it. To finish, I found this site is of some help: https://repairpal.com/subaru-forester-2010/problems