Update: I replaced the front bank O2 sensor with the proper one. Old one was somewhat carboned and discolored; it was obviously in need of replacement. Car now runs great, no longer sluggish. However, the gas smell persists. Since it's running fine and apparently not rich, I am guessing this could be caused by vapors under the hood coming in through the vents or HVAC? But I seriously can't find any fuel leaks anywhere - any suggestions on leak-prone areas to check under the hood? Are these things known to seep at such low mileage?

Also, the CEL came on once or twice, despite the apparent "normal" performance. After replacing the O2 sensor, I did disconnect the battery for several minutes in order to reset the diagnostics. However, I am doubtful that this was helpful, since the radio stored the station presets.

I haven't taken it to AutoZone yet. Is there a way to make sure the system was cleared? Can failure to clear it cause the CEL to come on again?

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280