I have a 2002 Chevy Impala, Engine light came on again for code P0171. Had the oxygen sensors changed a while ago. I was wondering if it could be from air in the cooling system. I had like a brown sludge on the radiator cap and I flushed it about 5 months ago, only the radiator, not the block and now I see the sludge again. I didn't know how to bleed the radiator so could air in the cooling system be causing this problem and code ??