Intermittent Auto Start-Stop May Need a Full Charge

Dear Car Talk:

After five years, I'm finally used to the auto start-stop feature on my 2017 Cadillac XTS. When I got him (yep, a him) home after a long stay at a body shop (sad story), the auto start-stop didn't engage for two weeks. All of a sudden, it worked ... periodically. Now, it works for a couple of days, then not for a few days.

How could a body shop have changed something like that? They didn't work under the hood -- to my knowledge! What's going on?

I love your informative and entertaining column! Thanks! -- Toni

I don't think they changed anything, Toni.

My guess is the car sat for a long time in the body shop without being run. That ran down the battery. By the time they repainted it and extracted all the chocolate chips from the muffin truck you crashed into, perhaps it wouldn't even start, and they had to jump it.

And if the battery is not fully charged, the auto stop-start system won't engage. It'll keep the engine running and the alternator turning at stop lights until the battery charges up completely.

If your battery is also old and weak (like from 2017, for instance), it may require more charging than a newer battery and that would lead to the engine running even more of the time.

The computer also will consider things like the engine temperature -- it won't shut off if it's still warming up -- and whether there's a heavy demand on the engine -- like if you're running the air conditioner.

So, lots of short trips tend to keep the auto start-stop from engaging because the battery hasn't had time to fully charge, and the engine may not be warm.

I'm guessing that nothing's wrong with your XTS, Toni. But have your battery and charging system checked and replace your battery if it's near the end. And then, as long as you don't get distracted again while a muffin truck is in front of you, you should be all set.

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