Is there a fob code to put the car into a battery saver mode ?

Yes. For the battery saving mode, press the unlock button twice while holding down on the lock button. The fob light will flash 4 times if you've done it correctly. Take it out of battery saving mode by pressing any button.

 

There's a youtube video ( https://youtu.be/c4hpQGxq4iY?is=FFv0vJG0EWKEriYy ) that mentions:

Battery guardian mode:
Key fob: Lock, then press lock quickly 2 more times. (Should get a different sounding confirmation tone/sound.)

Though I've never tried it.
(That video might be AI slop..)

 

That's for the fob.

 

didn't work for me.

I wouldn't mind accessing the diagnostic menu (not just the infotainment), but the odometer button trick didn't work for me. Neither did the temps/volts tip.

 

D'oh! I'm not aware of battery saving mode for the car itself.

 

Turning on the battery-saving feature of the key fob will also save your 12-volt battery too if you happen to wash or detail your vehicle with the key fob on you (or walk by your car often). Any active key fob that is registered to your vehicle in that kind of proximity kicks the car's electronics and pre-flight routines and pumps into a mode that is anything but sleeping to your 12-volt battery. Something I no longer forget to do with a reminder ...

 

Drop your keys into a Fariday bag to block it's signals. That'll also keep thieves from amplifying the fob signals and stealing your car; since some people hang their keys by the front door.

 

Are you talking about the 12-V battery? The car goes into deep sleep after shutting off, as long as the key fob is at least 3 feet away. No special procedure is needed.

 

A Faraday bag is not needed for the car to go into deep sleep. See my previous post.

 

Gen 2 Prius (2004-2009) was last to have a physical button on the dash to Enable/Disable the Smart Key System. Described as "If you do not intend to drive your vehicle for a long time, push the cancel switch to deactivate the smart key system".

These days if you want that feature --- P. 620 of the Gen 5 owners manual shows there is a customization setting to turn the smart key system on/off but it's not available until a dealer activates it. Then, I assume the option is visible to you to choose. My assumption there is backed by statements like P. 584 of the latest owner's manual talking about reasons your smart key may not work properly and suggests "Make sure the smart key system has not been deactivated in the customization setting. If it is off, turn the function on.".

Without going that route, know that there are multiple levels of 12-volt battery preservation and not just one level of sleeping which Toyota subscription technical documents mention under "Auxiliary Battery Saving" that kick in when out of range of the fob to "preserve the vehicle and key batteries" which is "activated when the vehicle remains parked for a long time". Such as after 5 days the "Door electrical key antennas and electrical key antenna stop emitting signals for creating the actuation area" and after 14 days "Door lock sensors and unlock sensors for seats other than the driver seat are disabled". Pages 202-203 of latest owners manual speak to this too.

Oh, and let's not forget if you choose to use a phone as a digital key you've now enabled bluetooth communications with your vehicle as another path to invisibly use your 12-volt battery. That's other electronics and antennas involved and listening which no doubt operate farther than the 2.3 foot range of the key fob antennas.