This is not uncommon, particularly in newer cars that have a lot of sensors to keep an eye on this sort of stuff. Fundamentally, the sensors have a finite lifespan, and after a few years they can simply stop working. When that happens, they don’t send the correct (or, in fact, any) signal to the car’s on-board computer which then assumes the engine has run out of oil or the windscreen washer bottle is empty and fires the appropriate warning light on the dashboard.
In some cases, you’re very wise not to assume the warning light is faulty, but if you’ve checked the oil level and the washer bottle is full, then you can pretty safely assume the sensor is kaput. The difference would be if the oil warning light is for oil pressure (not oil level). At that point, you’d really want to check the engine’s oil pressure independently at a workshop to be sure all is well. Oil level is not the same as oil pressure and an engine with a full sump can still have a lack of oil pressure which could destroy the engine quick smart.
The good news is these sensors are usually quite easy and relatively inexpensive to replace.
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We’ve asked Mercedes-Benz for help with your problem, but in the meantime you could take the car to a Mercedes-Benz specialist mechanic and have then show you, or do it for you.
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Buying a smaller, new car would give you the certainty of a long warranty and the benefit of capped price servicing. You could consider a Kia Rio, for instance, which would have a 7-year, unlimited kilometre warranty.
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