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Check engine lite on 1999 Mercury Sable

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The check engine lite keeps coming on says lean bank 1 cleaned the mass air,replaced fuel filter What could it be?
(1) Answer
Often times 'cleaning' the Mass Air flow Sensor is not enough. How is the fuel trim for bank 2?, especially Long Term. I wrote an article on PO171, P0174 at ::

http://repairpal.com/OBD-Code-P0171-and-P0174. Pay close attention to the Baro readings for the Mass Air Flow Sensor. If the Baro Sensor goes out, no amount of cleaning will ever solve that problem and Baro Sensors fail a lot. They are part of the Mass Air Flow Sensor. Also be sure that the both banks have very high Long Term Fuel Trim because, if one Bank is much lower, then the problem is a Vacuum leak, not a MAF Sensor.