Over the past 3 days, the keyless entry for fob 1 has stopped working. It unlocks the door when you push the button and it starts the car (red light flashes on fob). When I push the button to unlock the door and enter the car, the light on the side mirror flashes once but stays locked. I even held the fob next to the door handle.

Background: I purchased this vehicle used last month with 40k miles. The selling dealer (non-Acura) replaced the battery in fob 1 when I bought it. And it has been working up until earlier this week.

I read an article (non-Acura related) that suggest this could be a sign the car battery is getting weak. I'll check if it's still the original car battery; the vehicle has been in operation since June 2017. Has anyone experienced this with their RDX?

 

Update: This morning, it worked the first 3 times I I approached the car. The 4th time, it took 3 tries before it unlocked. Again, the unlock and lock buttons work; the car starts fine and settings match driver 1. Once the rain stops, I'll check the 2nd remote.

 

It does sound like a low battery in either the key fob or the car itself. Have you gotten the car battery tested recently?

 

It looks like the original car battery.

Had trouble first thing this morning (40 degrees) and the next 2 times. Worked more consistently this afternoon.

I decided to change the battery in the key fob. We'll see how this works.

 

Yah you probably should get the car battery tested. I would think that it should last longer, but sometimes that's not the case.

 

Update - Keyless entry worked all day after changing the key fob battery. Let's see what tomorrow brings.

 

Honda under hood batteries are not that good anymore. At three years, maybe more from the date first sold, you are on borrowed time with that battery.
As suggested above, get it load tested.

 

Wow, I would've expected around 5 years...

 

The OEM batteries are designed to get you out of the warranty period. My 2015 Camry LE battery failed at 2.5 years (replaced under warranty) and that had fewer electronics than the Acura. This vehicle has been in service since June 2017 so it could be time.

 

Battery life is completely dependent on many many multiple factors.
We have a 2016 RDX Advance and is still on factory battery since Feb 2016.
My 04 TL still had the factory battery in it when I traded it @ almost exactly 5 years.
My 08 TL-S had the factory battery replaced @ about 3 years under warranty. Warrantied battery lasted about 8 years.
My daughter's 2016 CR-V is still on the factory batter since July 2016.

YMMV

 

The batteries that go into the key fob remotes need to be high quality, name brand. That could be you only issue.
Perhaps the selling dealer cheaped out. You should still make sure the 12 volt is load tested. Dealers usually do that with most service.
If you are DYI for most maintenance, then a place that just sells batteries would probably do it, maybe for free.