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.

You could be looking at anything from a maladjusted or broken selector, to a transmission with low fluid level or even a transmission that has pretty much failed internally. All three possibilities can see a car with reverse and no other gears.

A transmission specialist is your first port of call, once you’ve checked the transmission fluid level yourself, of course. The owner’s manual will tell you how to do this.

That’s not a huge mileage for an engine like this, and I wouldn’t expect to see timing chain stretch over that distance. However, it begs the question of how the vehicle has been serviced. Timing chains really should last the life of the car (or, at least, the engine) but if the engine hasn’t been treated to the correct oil changes and general maintenance, the timing chain can be one component that will suffer.

If the car was bought second-hand, it would be worth going through the service history to see if any scheduled servicing was skipped. This could be the root cause of the problem. It’s also important not to confuse a stretch timing chain with a worn chain tensioning system. The tensioner holds the chain tight, taking the slack out of it and allowing for some stretch to be taken up over the years of operation. If it fails or becomes weak, you might start to hear the chain rattling.