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An exhaust leak on the driver side is your #1 suspect, which the O2 sensor reads as lean, and the PCM commands more fuel to that bank.
If you're running GM O2 sensors, failure to cause your condition is rare. I've found that 250k mile GM O2 sensors work just fine if your base tune is even close. They don't switch as fast as new ones, but don't consistently read high or low.
 
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