50K miles, 2012 xB got code P0137 for over one year and half. Most time just show pending or historic fault, Then after running a certain rides (2 weeks), the code will disappear. It used to take a long period for code to come back, now it comes back in couple of rides. I think the system self check the O2 sensor each time when start the engine, It fails because it doesn't reach the minimum voltage. So does the downstream O2 sensor fail? Do I need to replace the downstream O2 sensor? Or get a new battery? Or just add some engine clean fluid? Thanks.