P0121 and Hybrid Battery Warning

You may want to edit your post (or ask the moderator to do so, if your edit button is gone already) to say P0121. It's right in the title, but typo'd in the post itself, which could make it harder for others to find with a search.

This is probably a good place to link to the post about which codes are throttle sensor and which ones are pedal sensor, because that's a recurring source of confusion in the Prius. You got it right here, though; your code is about the throttle sensor.

The fail-safe response for the ETCS codes is supposed to be:

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If you keep driving after that happens, you're using only battery power to move the car, and there is only enough in the battery for a quite short distance at low speed before you'll get a warning about that.

 

The available distance to drive on the battery is really quite short. So when the engine's disabled, if you keep driving, you reach low-battery-warning condition really very quickly.

There are threads—plural—here promulgating the idea that the power-steering light is what tells you you've run out of gas. Some people were deliberately running out of fuel to see what would happen, the engine would stop, the car would keep going on the traction battery, it would deplete, the DC/DC converter would quit producing 12 volts to conserve traction battery charge, the 12-volt battery would deplete, and the steering ECU would light a warning for low battery voltage. (It turns out to involve a weird design decision by Toyota, where there is a trouble code set for stopping the engine because the fuel is out, but they decided to have that trouble code not light any warning light; that's why the first light you might get could be so far down the chain of consequences.)

Also, I don't know exactly what warning you saw. You described it once as a hybrid-battery warning, and once as a hybrid-system warning. The master warning light (a triangle in most models, they used a different shape in Prius v, I don't know what it looks like in Prius c) is generically about any problem with the hybrid system, not necessarily just the battery.

I think you can get the hybrid-system warning if the code P0A0F is set, and that code is set when the power management control ECU (more or less the orchestra conductor for the car) signals the engine to run and the engine doesn't. So if that code has been set (which wouldn't be surprising), that could explain what you see on the dash.

There are many code scanners that can't communicate with all the ECUs in the car, so they might show you whatever codes are in the engine control module, but not show any codes from the power management control ECU. That can be a common explanation when someone is able to retrieve an ECM code (like your P0121), but is not seeing any hybrid-control codes, and yet a hybrid warning light is on.