Question on similar obd trouble codes

My Taurus has been showing 171 and 174 for a while but now it has added 1131 and 1151. All of these indicate lean mixture on both banks. What is the difference?

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P0171 and P0174 indicate that the adaptive fuel strategy has reached a preset limit... typically, this would be when long term fuel trims have reached 25% fuel adjustment (rich or lean, though lean is more common than rich).
P1131 and P1151 indicate that we have moved beyond the fuel systems ability to cope with the condition and the O2 sensors are no longer switching rich/lean and are, in fact "trapped" lean.
The P0171/4 are telling us that we are getting into trouble - let's do something about it - many times with no driveability symptoms. P1131/51 are telling us that we ARE in trouble and there is every chance that we are experiencing driveability symptoms.... or soon will.
Compare this to a small child.... P0171 is "I have to go baffroom.". P1131 is "I have to go baffroom" along with foot stamping and breath holding.... If we wait too much longer than this.... it can get less pleasant.
HTH

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