Active Blind Spot and Eco Mode Inoperative; Battery Testing
 

We just had our first serious amount of rain in California in a long time, and a couple days ago I noticed a console message in our 2014 E550 saying "Active Blind Spot Assist Inoperative" when first starting the car. I found various posts talking about how the radar sensors in the rear bumper could go bad, get corroded, etc. I did however notice that even though this message shows up when starting the car, the blind spot assist seems to be working. The orange triangles in the side view mirror light up between 0 and 20mph and they go red when a vehicle is passing on either side.

While investigating further I also noticed that our Eco mode is not functioning (on/off button is not green on start of the car and the button does nothing; console shows its not active). In addition, I believe the left/right turn signal lamps on the side view mirrors aren't lighting up. I've read that the blind spot assist and the turn signal lamps are on the same fuse so decided to look there. But then without touching anything, when I started the car, everything's working again.

Then I ran through a few cycles including cold starts, warm starts over a couple days and found that about 1 in 5 starts, everything is fine. But 4 out of 5 times, my blind spot assist, eco mode, and turn signals are not working.

Further reading suggest perhaps the batteries. My vehicle is 5 years old (original owner). The main battery has never been replaced. The auxiliary battery was replaced 6 months after I purchased the vehicle as the dealer found some type of malfunction with it. The car was driven about an hour this morning across two starts. With the key removed from the car and the engine off, I tested the battery voltage on both batteries. The small auxiliary battery in the trunk next to the spare tire reads 13.76 volts. I would have expected something in the 12.x range so that suggests the car is still on and actively charging the aux battery?? The main battery in the engine compartment I tested using the test leads (sliding red cap and the ground bolt near it). It reads 12.02 volts.

This is leading me to believe that my main battery is shot and that the rain is a red herring ... coincidence timed with the age of my car at 5 years.

But what's confusing me is whether this car with all of its electronics cannot be accurately tested for voltage until some cool down period or inactivity period? Reason I ask is the 13.76 volts on the aux battery. Is there some kind of idle time I need to wait for before testing these batteries for voltage? Or are my readings correct and is my main battery shot?

Thanks!

Disconnect the aux battery and test again. ECO start/stop and blind spot, along with other driving assist functions are controlled by that aux battery. I suggest you replace that battery as it is more than likely flat-lining on you.

Thanks for the reply. I'll disconnect and test the aux.

^ yes, that is also powered up by the Aux battery in the trunk.

Disconnected the aux battery and checked the voltage and it's at 12.56v. That seems normal to me. The primary battery is now at 11.98v. That seems low, would have expected 12.5-12.6 since it was just driven for 30 minutes.

Given the systems (eco, active blind spot assist, turn signals) work from time to time, I don't think it's a fault in any of them. Seems like the primary battery although those subsystems are more related to a bad aux battery no? Since the car's 5 years old and that primary battery voltage seems low, maybe I'll swap out the primary first. And if that doesn't help, I guess I'll swap the aux. Or maybe I should just do both.

Ok, I'll just replace both. It's probably time anyway. Thanks.

Swapped out both this morning with batteries from Autozone (3yr warranty). H7-AGM for the primary and AUX14 for the auxiliary. Started the car and drove four times today and all systems are functional again with no error messages. ECO's working and so is the blind spot assist. Will keep my eye on it for the next few days.

As for the old batteries... tested them again out of the car. 12.3v after 60 minutes of driving on the primary (which I believe is not good). And 12.56v on the aux which seems fine. But probably doesn't mean much without a load test since it could be that it drops on load. Another theory is that the primary is in bad shape and drops on load on start preventing various systems from properly initializing.

With the new batteries, hopefully the issues don't come back.

It's been over a week. Just wanted to report back that replacing the batteries did the trick. No more issues.

Nice!