Long-Term Mercedes-Benz C300: Keyless-Go to Keyless No at 6,700 Miles
Cars.com’s long-term 2015 Mercedes-Benz C300 4matic test car has been giving us the business recently. More accurately, the C300 has been giving our local dealership plenty of warranty business with its string of issues. The latest issue happened 150 miles from home in Elkhart Lake, Wis., where the Keyless-Go key fob with keyless proximity entry and push-button start threw in the towel at 6,700 miles. A dead key fob battery wasn’t the problem.
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Unlocking and starting the C300 without a functioning fob requires pulling the manual key from the fob, twisting a small, easy-to-lose cap off the door’s lock and then using the manual key like a traditional twist-and-turn key. If this happens to you, hopefully you’re not in front of a banquet hall full of industry colleagues like us, because you get an earful of car alarm once the door is unlocked manually; the alarm turns off once the car is started. To do that without the functioning push-button start, the ignition start button needs to be removed by pulling outward; the key fob fits into the slot behind the start button to start the car.
Don’t be alarmed if you can’t lock the driver’s side door using the interior door lock buttons; that’s intentional to keep the key from being locked in the car. The workaround is to close the driver’s side door and then reach through the passenger side to lock the doors.
We snagged the second key fob back at our offices in downtown Chicago to see if only one key fob was toast, but the second one also didn’t work so we headed to the dealership. The problem, as it turned out, was from a glitch within a control module that handles the Keyless-Go operation. Our dealership’s service consultant said something went corrupt in the module, so a technician flashed the control unit with the latest software and we were back in business. The update was covered by our warranty. A few thousand miles later and we’re still in good shape with a functioning Keyless-Go system.
