When I saw the RPM to zero it reminded me of Ford C Mxax Hybris we used to owned. Where you feel the engine stop working and then start up once you accelerated.
When the van did this I could not tell if the engine stopped running or not.
From afar, I don’t think that anyone can give you a good diagnosis unless you can figure out whether the engine is running–or if it isn’t running–when the tach shows zero. But, in order to rule out the possibility that this situation is just the “normal” engine stop/start cycle, you should temporarily disable the auto stop/start by pressing this button.
The placement of these buttons varies from model to model, but the symbol is the same. Remember that you need to do this after each time that you start the engine, as you can’t disable it permanently.
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OP can’t tell if motor is stopped. Zero tach could be clue though. Have someone look at motor while stopped. Belts turn?
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The car is smart enough to know that it shouldn’t auto-stop the engine if the battery system may not be robust enough to start it again. You probably had a weak battery when you bought the car.
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I think Asemaster got it right. Now that you replaced one of the batteries I still am of the opinion you should replace both) the start/stop is now functional.
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Maybe that’s where the question posers live.
Don’t open the hood when doing this test. I’d say once the car knows the hood is open, the stop\start function may deactivate. Also there should be an indicator somewhere in the dash stating when your engine is not running due to stop\start feature.
Wouldn’t that be the RPM gauge?? lol
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As a test, leave hood popped. Motor will never shutoff.
And I had my post removed for suggesting to read the damn owners manual. Bunch of “snowflakes”
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It’s all in how you phrase it. Good idea, bad presentation.
I wonder if the OP will return to tell us if the problem is the Start/Stop function now working?
After 4 days, I doubt if he/she will return to clarify anything.
Maybe the person you told this was offended by your demeanor. I’ve never been told something like that while seeking help in person but I’ve been told that many times on the internet.
Courtesy and friendliest go a long way.
Suggest to just ignore. It is not a reflection on you.
You know what else also goes a long way?
Accepting the advice that knowledgeable people give you and TAKING that advice . . . versus repeatedly asking the same questions and making the same comments, desperately hoping that someone will tell you what you want and/or need to hear 
That wasn’t directed at one single person, btw
About 500 pages to the manual for that car.
Not unusual. They all have an index.
500 hundred pages , so what ? It has an index so you can just look up what ever you need.
Cigroller beat me to it .