Monday, November 16, 2015

Toyota Prius: 12V auxiliary battery check

For 2004-2009 Toyota Prius, you can check the 12V battery's status in diagnostic mode.

To enter diagnostic mode:
1. press the Power button (without pressing the brake)
2. press and hold the Display button (on the side of the dashboard computer)
3. tap on the yellow highlighted spots, 1 -> 2 -> 1 -> 2 -> 1 -> 2. There's no visible buttons, you just need to tap on the two spots as shown below:
4. if done properly, it should enter the diagnostic mode. Alternatively, you can press and hold the Display button, and then toggle the head lights ON/OFF 3 times instead.

To check the 12V battery's status:
1. tap Menu > Display Check > Vehicle Signal Check
2. a good battery should be around the 12.x range. If you turn on the vehicle, it should start charging the battery and the value would increase to 14.x.

To exit diagnostic mode:
press and hold the Display button for 5 seconds

Owner manual:
You can download them at
(It's a shame that they are not available on, their Technical Information System (TIS) is down or no longer available)

How to see if your 12v Auxiliary Battery is going bad - YouTube
Quick test of a Prius 12v battery - YouTube
Toyota Prius: How to Check the 12v Auxiliary Battery Health -

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