Weird stuff happening? Test The Battery: Here's How The procedure below is for 2004-2009 models. Others must check the battery with a voltmeter or a scan gauge. If anyone has a verified procedure for use on other models please post it. It's best to eliminate the common, simple and (relatively) cheap causes first. Dying batteries are near the top of this list. 12V car batteries have a lifetime of a few years. Draining a battery will usually permanently degrade it by some amount, and it will never work as well again. If the car starts having strange symptoms, or doesn't start reliably but runs fine, or if MPGs have been dropping and you've eliminated the other simple causes (cold weather, low tire pressure, lots of recent short trips), test the 12V battery. A failing battery can even reduce MPGs because the car spends more energy trying to keep it charged. A pass/fail battery tester, even at Toyota dealers, sometimes does not correctly test a Prius battery because the passing voltage threshold on the tester can be too low for reliable Prius operation. Low voltage is often OK for a conventional starter but computers can get confused. You can easily test the 12V battery yourself. With a voltmeter measure either at the jump point under the red cap inside the fuse box under the hood, or at the battery itself. With the car OFF, if the voltage is much less than 12.0V it means that the battery is dying. DO-IT-YOURSELF TEST PROCEDURE (thanks to jdenenberg): - Without brake pedal, press the Power button once and release to enter ACC mode - Press and hold the MFD Info button, then turn the headlights on and off three times to enter Maintenance mode; release Info button - Press “Menu” (on screen) - Press “Display Check” - Press “Vehicle Signal Check” - the battery voltage is shown and should be about 12.4 to 12.8 Volts (normal for an unloaded battery) - Again without brake pedal, press Power button and release to put a current load on the battery - the voltage should stay above 12.0V (if less than 12.0V the battery is not well, or there is a fault or unusual load somewhere) - Press brake pedal and press Power button once to enter "Ready" mode - the battery is now charging at about 14V (if less than 13.6V or more than 14.4 there may be a problem with the charging circuit) - Turn car OFF to leave Maintenance mode For an alternate procedure that does not require turning the headlights on-and-off see Entering Maintenance Mode (aka 12V Battery Check) without Switching Headlights ON/OFF | PriusChat If these procedures cannot begin then either the battery is dead or there is some more serious electrical problem. If the 12V battery is dead you're in luck because replacing it is one of the easiest and cheapest-to-fix electrical problems there is in a Prius. I have heard of one direct drop-in aftermarket replacement, the Optima YellowTop Deep Cycle Battery model #S46B24R. One mail-order source is eLearnaid (see Prius parts ). All others require modification to one or more of their terminals, mounting hardware, vent tube, or cables. If the Smart Key System fob no longer works well, test the fob battery. Press any button on the fob. You don't have to be near the car. A small red LED should flash in one corner. If not, dying/dead fob battery. However even if it passes this test, the battery may still have weakened enough that it does not work as well as it did originally and may need to be replaced. Also note that nearby metal or electronics such as coins or a cellphone in the same pocket or purse can interfere with fob-car communication. If the 12V battery is low you can try using the Prius itself to recharge it by turning the car fully ON ("Ready", as though you were going to drive it) and letting it sit with the shift in "Park" for nine hours. It can take that long for a full charge because the battery is rated at 36 Amp-hours, and the system is limited to 4A. Of course be certain that the car is physically secure, and, if indoors, has an adequate air supply.