Code 91 means that your drivers side O2 sensor is reading lean. Since the right side is reading normal, that rules out a vacuum leak or a fuel pressure problem. Likely causes are a bad O2 sensor, or a bad injector in cylinders 5-8. If you don't have a miss, it's the O2 sensor. If you do have a miss, do a power balance test, and see which cylinder it is.
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