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.

Not really, and modern cars have proven they should be easily able to cover this distance with plenty of life left in them. That, however, is on the proviso they’ve been serviced and maintained correctly.

Even the best, most robust cars can fail long before 169,000km if they’ve been neglected, particularly with regard to oil changes. So the real answer to your question will depend on the car’s service history. Check the history carefully for any signs of skipped servicing. We’d buy a high-mileage car with a full history over a low-mileage example with no records every time.

Unfortunately, the only simple solution to your problem is find the little circuitboard (chip) that went missing when you did the battery replacement. Without that, the key can’t talk to the car’s computer and let it now that it’s okay to allow the engine to start.

The alternative is to find a car locksmith and have a new key programmed to talk to the car. Exactly what else this will involve will depend on the car itself and how the security system is configured. You may need to change other components on the car and it could get pretty expensive. You may also need to trailer the car to the technician.

If that doesn’t pan out, perhaps try a Mercedes-Benz dealership which should be able to sort it out and get the car running. But you’ll be paying full price for that service.