Don't Let Stopped Stop-Start Feature Stop You From Living Your Life

Dear Car Talk:

I love your column and always get a laugh from it.

I have a 2018 Mercedes Benz C300 with 21,000 miles. Yes, I am a little old lady! It has that gas-saving feature where the engine shuts off when you stop at a red light.

Well, at first, it really annoyed me, but now that I've gotten used to it, it has suddenly stopped stopping!

It's been over two months, and it hasn't shut off once. Can I just ignore that? Or am I asking for trouble if I don't take it in? -- Sharon

Continue to live your life, Sharon. This does not require an emergency $1,200 trip to the dealership.

There are several possible explanations for why your automatic stop-start system stopped stopping. The most likely one is that the car's computer has decided the engine needs to run more. Why would it decide that? Is it in cahoots with the Saudis to keep oil prices high? No. Well, not that I know of.

It's just that sometimes the engine needs to run for a variety of reasons. For instance, if your battery is not fully charged, the computer will keep the engine running at traffic lights in order to keep charging the battery.

With only 21,000 miles on the car since 2018, it sounds like you don't drive a lot. And if most of your trips are short, the engine may need the whole trip just to get the battery fully charged. Or, the battery could be weak, and wearing out, requiring more charging time than it did during the first few years you owned the car.

The stop-start system will also be suspended if the engine hasn't fully warmed up yet. So short trips can affect that, too.

If your climate control system is working hard to heat or cool the car, the engine will also keep running. So you may notice the stop-start feature activates less in very hot or cold weather.

There are a couple of other explanations that I really don't think apply to you, Sharon. There's an on-off switch for this feature (it's marked with an "A" with a circle around it). It's possible to hit the switch by accident. But the feature turns itself back on each time you drive the car. So that's unlikely.

Also unlikely is that you're driving your car in "sport" mode. That mode -- among other things -- disables the stop-start system, so you won't lose time when you drag race with BMWs. Like I said -- I don't think so.

So, I think it's nothing to worry about, Sharon. I'd get your battery tested, just in case. And if the battery is good, I'd just keep driving. The stop-start will come back when it's good and ready.


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