TOYOTA 2011 Avalon - Manual (Page 451)

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4-3. Do-it-yourself maintenance
AVALON_U (OM41428U)
Key battery
Replace the battery with a new one if it is discharged.
n You will need the following items:
l Flathead screwdriver (To prevent damage to the key, cover
the tip of the screwdriver with rag.)
l Small Phillips-head screwdriver
l Lithium battery CR1632 (vehicles with smart key system),
CR2016 (vehicles without smart key system)
n Replacing the battery (vehicles with smart key system)
Take out the mechanical key.
Remove the cover.