I would replace the Oxygen sensor as they are supposed to be replaced every 60,000 miles. Doubt that you would do any harm by driving your 420SEL a few hundred miles before doing so, considering that some folks drive a car with the CEL light "on" for thousands of miles because the engine behaves normally, and they get used to it being "on".

As you probably know, replacing the Oxygen sensor is a simple DIY task with the crow's foot tool.

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Fred Hoelzle