1999 Mystique is running lean.

[Jack Flash]

this Mystique throws a random bank 1 lean code, I've tried all I know...vacuum hoses, gas change, new thermostat...
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seems that the only time the check engine code pops up is when I drive the car, stop, shut it down let it set for a few minutes...then when I crank it up the code pops up. never happens when I go to work & the car sits for 4 houre..just short stop & go trips.
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any ideas ?

 

[BrooklynBay]

What's the code number? You might have a defective O2 sensor. This once happened on my 1995 van. I would get a random code with an intermittent check engine light. The sensor was borderline from being out of spec.