Key Fob Low Battery - Very Strange Symptoms

My 2012 Prius Three started exhibiting very strange symptoms when attempting to lock the car

Expected behavior:

1) Click the "lock" button on the key fob
2) Lights blink once, single beep emitted, doors lock

Actual behavior:

1) Doors make a sound like they're locking but there is no blinking light or sound emitted
2) After 1 second it makes a sound like the doors are locking or unlocking, I can't tell which. Also no blinking lights or beep.

After this all the doors are locked except the driver side door. When I press the Lock button the doors respond immediately but always after 1 second there's a second mechanical lock/unlock sound and the driver side door remains unlocked. Very occasionally the first response (first mechanical clicking) is accompanied by either a beep or blinking lights or both but followed by what I assume is the driver's door unlocking.

I experimented with using the sensor on the door handle to lock and it exhibited the same behavior as using the key fob.

Then I took out the physical key and using it did NOT exhibit the behavior -- i.e. with the physical key I was able to lock the door with no trouble. Note that even with the physical key, locking and unlocking are accomplished via electronics and a relay, not a direct mechanical connection from the key to the lock. This seemed to rule out a mechanical issue with the locking system, leaving only a defective key fob or low battery.

Sure enough, the battery voltage was down to 2.88v, and replacing the battery cured the issue.

I want to understand what the car was doing and why. One possible explanation is that, in the presence of a low battery, potentially about to fail, the car decides not permit locking the driver's door, to prevent you from being locked out, and also as a warning. I don't remember seeing this documented anywhere though, and I never got any warning on the display or dashboard.

 

I replaced the 12v battery last year, so that’s not an issue here. Curious that I never got a message on the display about the key fob battery.

 

While I agree with you on the symptoms, all I can do is describe the events that transpired.

The other key is (and has continuously been) in my desk drawer at home, while the behavior described occurred at various locations far away from my house. Also, as soon as the key fob battery was replaced the problem stopped and has not recurred.

 

So, the problem recurred yesterday, 10 days after being "cured" by replacing the key fob battery. That battery was definitely low, but clearly the problem is elsewhere.

I checked the 12v battery with both the onboard diagnostic mode and a multimeter. With the ICE off and no load it reads 12.2v. With maximum accessory load (high beams, rear defroster, fan on high, dome light) the voltage drops to 11.0v, which is strongly suggestive of the 12v battery being on its last legs.

Guess I'll have to file a warranty claim, as the battery's only 1 year old.

 

Curious. In the Gen III the mechanical key seems to engage the lock motor, which doesn't make sense if it's going to work when the battery is dead. Maybe it uses the motor if available but also has a physical linkage?

 

To complete this thread, the problem turned out to be a failing door lock actuator. The problem slowly got more frequent and I was finally able to reproduce the issue with the lock/unlock button on the driver's door console, eliminating the key fob completely. Yesterday it started randomly failing to UN-lock the driver's door after successfully locking.

I took it to the dealer and they quickly diagnosed the driver's door lock actuator, which they replaced.

Seems to be back to working again.