If a toyota highlander keyless entry is not working, what is the cause?

By far the most common cause of a keyless entry not working on a Toyota Highlander's going to be a dead battery in the FOB itself. First, let's go ahead and rule out other problems. If You've got a second key FOB, just try using it; does it work If so, we don't need to be concerned with the car's electrical system or locking system and know it actually is just the remote. Now, it's always possible that the FOB itself has become damaged; if that's the case, You're looking at around $75 to buy an equivalent HYQ14AAB smart key, though after-market ones with less capabilities are available for a quarter of that. Most likely, though, You just need to the 3 volt lithium battery in Your remote; a 2302RC of any brand should fit and work for most Highlander years and trims.