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Left Foot Braking and Subaru's Brake Override System
All newer Subaru vehicles are equipped with Brake Override System or B.O.S. This is a safety system is designed to prevent unintended acceleration and loss of control of the vehicle. Subaru implemented this system so that in the event that something occurs which jams the accelerator pedal (incorrect floormat, trash on the floor, etc.) that simply applying the brake pedal will stop the acceleration of the vehicle and allow the driver to maintain control. This has been an important safety innovation, which has now been adopted by most other automakers.
Your Subaru constantly monitors a myriad of sensors and switches and knows when you press any pedal; brake, accelerator, or clutch. If the vehicle sees that you have pressed the brake and the accelerator at the same time the B.O.S. system activates and engine power is greatly reduced to idle speed as a safety precaution.
Some customers that drive using their
left foot to depress the brake pedal will notice an uncomfortable
sensation when approaching near-stop intersections or turns. They
will perceive that the vehicle is about to stall, when in fact, it is
simply the B.O.S. doing its job.
Since driving is such an integral part
of our lives many drivers don't even realize that they in fact use
their left foot for braking actions.
Some common instances where the Brake Override System may engage and reduce engine RPM's to idle:
- Driver is approaching a tight turn, such as a side street or driveway. The customer brakes to reduce speed for the turn (using their left foot) and then attempts to accelerate as they enter the turn before their left foot has been removed from the brake pedal. The vehicle detects inputs from the throttle and the brake, and brings the engine down to idle speed.
- Driver is approaching a traffic signal and slowing for a stop, but then the signal changes as they are still braking. The driver applies throttle to drive through the intersection prior to removing their left foot from the brake pedal completely. Once again, B.O.S. engages and brings engine down to idle speed.
- Driver is stopped on a steep hill, using their left foot to hold the brake pedal. They use their right foot on the accelerator to attempt to move the vehicle up the hill, but when the Brake Override System detects brake and throttle input at the same time engine speed is reduced and if the driver has reduced braking force, but not totally removed their left foot from the brake, the vehicle could even move slightly down the incline.
If a driver experiences what they perceive as a near-stall condition in a situation similar to the ones listed above, they should pay very close attention to their driving habits, as the majority of these situations are traced to B.O.S. application rather than a vehicle malfunction.
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