Replacing Remote Battery

Do a search on this site. There are DETAILED step-by-step instructions for replacing the 12V battery on the Prius.

 

Sounds like you have an aftermarket alarm installed. I suggest that you finish replacing the fob battery. Then use the fob to deactivate the alarm.

If that will take too much time, then open the hatch, find the 12V battery in the right-rear corner of the hatch floor, and disconnect the 12V battery negative terminal cable, where it connects to the body. That should disable the alarm (as well as the car itself.)

 

Just open the fob, remove the old battery, install the new battery, reassemble.

 

It should not, unless you are accidentally depressing the red panic button on the fob.

 

Not sure if this will work, but I hope it does. Try holding the fob right on top of the power-on button in the car for about a minute. On my previous car, an '06 Camry Hybrid, I let the battery completely die in one of the fobs for over a week and when I replaced it, it would not lock, unlock, open the trunk, or anything. I called the dealer to see how much they would charge to reprogram it. The service manager said to first try holding it right on top of the power button for a minute or so. Believe it or don't, it worked! And saved me who knows how much money too....

 

If the car will recognize the fob's unlock button then it should recognize the fob's lock button.

Make sure that the car is ready to be locked i.e., the doors and hatch must be fully closed. Check the fob to see whether there is something wrong with that lock button. Also, if you have the other fob available, see if that fob is fully functional or not. Then maybe you can decide whether the problem is with a fob or with the car.

 

If the unlock button works, then the systems on the car are probably working ok. Since you've already identified a problem with the alarm button on the keyfob, the lock button is highly suspect. One thing you could do purchase another keyfob off eBay to get a keyfob case in better condition. Transfer the circuit board and transponder from the problem keyfob to the eBay one. That way you keep all the programming as is, while getting a new set of buttons.

Replacing the case will work if the problem is with the rubber membrane that includes the buttons. Another possibility is that there is water damage to the circuit board (especially the button switches). That might be correctable by spraying the board with contact cleaner.

To open the case, first take it apart like you do to replace the battery. Then use a pair of needlenose pliers inserted into the mechanical key area to spread the case apart. The glue is weaker than the case (well, mine was...), so the case should come apart intact. The circuit board is just press fit into half the case, although you may need to clean some of the glue off. Be sure to transfer both the circuit board and the transponder chip to the new case.

If all this fails, you're probably looking at a replacement keyfob and associated programming.

EDIT: All this assumes a 2004=2009 Prius. I haven't seen the inside of 2010 keyfob.

 

If the alarm button was stuck/defective then perhaps the lock or unlock buttons are also.