My mother has a 2000 Ford expedition the Radiator blew 3-4 months ago and the CEL came on. I put my scanner on it checked it and it showed a P0171 and a P0174 (bank 1&2 lean) so i cleared it and told her to let me know if it came back well the other day she came to me and said it was on (and had been since the day after I had cleared it) well i swung by a buddys shop and he put his big High dollar computer on it and said that looked like Bank 2 down stream O2 was bad, So i changed that Yesterday drove it a few miles nothing came on so i dropped her car off to her and she drove it to work and bout halfway home (aprox 40-50 miles) and it poped a code so i swung by this afternoon and pulled the code excpecting to find P0171 again but instead it was P0174..... Any Ideas why changing bank 2 O2 would clear up the Bank 1 code but not Bank 2? and any ideas what else could be causing the code? Im not off again tell next monday so She is gonna take it to my buddy's shop prolly Friday and see what he can pull then.
Usually its the up streams that sense the problem, 9 times out of 10 the o2s are fine, they are reading a vacuum leak, I would look into a air leak after the maf, which includes the intake tubing, IAc line, pcv valve and rear elbow, intake gaskets