While I realize most here disable this “feature” as soon as they can, I’m a bit OCD about things working. The ECO button lights up, and message center reflects correct status. However, the motor never shuts down. Looked in the manual for fuses to check, came up empty. Dealer where I bought the car said they replaced the auxiliary battery. Any thoughts?

 

The ECO feature will only stop the engine if certain conditions are met. If the battery isn't sufficiently charged it will keep running. I have a single-battery model but assume that both batteries would need to be fully charged. If the engine is under load, e.g. A/C on full, it won't shut off either.

 

Yep, if the main battery is even a little poorly then the SS system won't work even if the secondary battery is completely healthy and fully charged.
It's a deliberate design to prevent the situation where the SS system shuts the engine down but the main battery doesn't have the necessary oomph to start 'er up again.
What many owners don't understand is that the secondary battery isn't used to start the car / power the starter motor, that is still done by the main battery coz it takes lots of amps to power the starter motor, more than the secondary battery has if used repeatedly. The secondary battery is used only to power the peripherals such as HVAC, lights, infotainment etc once the engine has been shut down by the SS system, which preserves the power of the main battery. Then once the engine starts up again the main battery and alternator take over and the secondary battery goes back to sleep.

 

That's as I expected, but I didn't want to give incorrect information.

 

So, I assume a trip to O’Reilly Auto Parts for a battery load test?

 

You should invest in a battery tender/maintainer, CTEK is popular around here, it will tell you if the battery is too far gone to save and if not it may be all you need to get the main battery up to scratch again and get the SS working. If that doesn't work then yes time for a new battery and in any event a battery tender is a must have for any F-Type.

 

I disconnected the eco from working, no annoying stop start, 2 minute job disconnected in boot, light stays lit on button all the time but no error messages, love it

 

Mine stopped working shortly after I acquired the car. battery was 5 years old at that point. Similarly, my mom's GLC43 AMG had the same thing happen around ~4 years. It is just a symptom of battery degradation over time. Charging to full with a tender does not help, as the float voltage is likely still too low.

Eventually, I want to replace it with a Li-Ion battery when it truly dies, and I am hoping eco start-stop can coexist.