98/99 Windstar DTC - 0171 & 0174

The van belongs to a friend of mine. DTC 171 (lean, bank 1) has been latching on and off for some time now. It's an intermittent thing. You can reset the code and it may not come back for weeks. Once in a while, DTC
174 (lean, bank 2) will latch as well. This happens far less than the 171. Is this more along the lines of an O2 sensor going bad, or may something more be amiss?
Thanx in advance for any advice!
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Additional info... This only seems to happen while starting the engine. An 'improper' start (i.e. the engine balks while starting) will set the light. It doesn't come on during normal driving. This has also happened to my '02 Taurus once.

DTC
171.
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