Upstream O2 Sensors? 2001 E150 XLT 5.4 200K Miles

[leroge]

Getting codes...( Autozone diagnostics )...

P0171 - Fuel system too lean ( cylinder bank 1 )
P0174 - Fuel System too lean ( cylinder bank 2 )
P0153 - HO2S-21 ( Bank 2 sensor 1 ) circuit slow response

Multimetered each upstream O2 sensor....each getting 11+ volts, heating element 3.4 ohms resistance and each producing fluctuating voltages from .2 to .80 after 10 minute idle.

Thoroughly cleaned Mass Air Flow ( MAF ) Sensor with CRC cleaner.

Autozone reference says problem most likely a bad MAF for this vehicle.

Ideas????

[leroge]

....and the problem was.....

Brought my E150 to an emissions shop who did a vacuum system 'smoke test'. Sure enough, the EGR was emitting smoke badly.....so the suspected problem was an EGR vacuum leak. However, it was suggested I replace the fuel filter. Sure enough it was really cloggged. Replaced it. Replaced the EGR myself...a real pain to do. 'Codes' gone. So, it appears either the EGR alone or the fuel filter alone, or some combination of the two, was the cause.

FYI