The CEL light on my '99 ML430 with 98,000 recently came on and the OBD generated a P0156 code (driver's side O2 sensor, after the cat I believe). Refueled with 93 octane, put the cap on and then noticed the CEL was out. 20 miles later it was on again. Refueled again a few days later, made sure the cap was tight (also did visual inspection of seal and could see no deterioration) and when I started the engine, the light was out again. That was 100+ miles ago and the light is still out. Anyone care to guess about whether I'm dealing with an O2 sensor ready to fail or combination of fuel cap and octane? Thanks.

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Jim