I have a 2002 Prius that has 200k+ miles on it. Recently the check engine light came on. Not thinking anything of it I drove it home to get it looked at. While driving it home I noticed that the car would want to automatically brake on the freeway till I came to a complete stop. The brake light would be on as if the emergency brake was engaged. Prior to this I had taken it to the dealer and they said the battery was dying, however I replaced the battery for about 2k and the problem still persists. The check engine light is also on. Any information would be helpful.
Was it the correct battery? at that age it may be the 2k$ battery, ie the 10 year minimum battery life has been surpassed.
Get the DTC codes read. There may be a code for the regenerative braking system that is causing your symptoms. Or a faulty traction motor causing the drivetrain to drag. Either way, the trouble codes are the first step.
On the Prius the brake system is operated by the Power Control Module in ordinary driving conditions. When you step on the brake pedal, the pedal feel is artificial. There is a brake pedal position sensor that instructs the PCM how much retardation to apply. The PCM first engages the regenerative braking. As commanded, the friction brakes of the front calipers and rear units are applied using the ABS unit and pressure to bring the car to a faster stop.
I think your problem is signeled by the ‘brake light’ being lite. The ABS unit may have a problem where a solenoid valve is not operating correctly. Or, it could be the brake postion sensor is sending an erroneous signel to the PCM. A equiped technician should be able to read the various sensors and command the ABS solinoids and functions which should zero in on the fault.
BTW, have you flushed the brake system a few times in life of this car? ABS units do not take kindly to degraded brake fluid.
Hope this helps.