2010 Prius Error Code P1578 & P0571

any symptoms?
what code reader?

 

The switch on the brake pedal is really two switches. One of them only closes when you press the pedal (good for controlling the brake lights). The other is normally closed and only opens when you press the pedal (good for canceling cruise control). The power management control ECU monitors both of them: the first at the STP terminal, the other at the ST1- terminal. So it would normally never see a low voltage at both of those terminals at the same time.

P0571 means it saw both STP and ST1- below 1 volt for at least half a second at the same time.

For P1578 you probably have the wrong fortune cookie entirely. P1 codes are manufacturer-defined and mean different things in different cars, so the risk with just googling for them is you get nonsense like "ETC power less than demand" which is undoubtedly the right fortune cookie for that code in some completely different car.

In a Gen 3 Prius, the fortune cookie for P1578 is "brake system malfunction", and what it really means is the power management control ECU got a message sent to it over CAN from the brake ECU telling it of some brake system issue, and cruise was on when that message was received.

Where you find information like this is in the repair manual (more info) which also has troubleshooting workup steps and circuit diagrams to help solve the issues.

My hunch would be to focus on the P0571. A simple issue with the brake pedal switch or its wiring could have caused that code directly, and if it also was detected by the brake ECU, might have elicited a CAN message that could have led to the P1578.