What’s the cause of sudden acceleration in a Subaru Legacy?

Question: Sometimes, when I take my foot off the accelerator and put it on the brake, my car, a 2019 Subaru Legacy 2.5i Limited, accelerates suddenly. It stops only with a hard push on the brake. After this happened several times, in February 2019, I took the car to the dealer and left it for three days to diagnose and correct the problem. The dealer could not find the cause, even after I went for a drive with their director of after-sales operations. His conclusion was that, inadvertently, my foot remained partly on the gas pedal when I transferred to the brake pedal because I was wearing heavy winter shoes. I was very doubtful but I had little choice but to accept this verdict … This morning, the same thing occurred twice …

Answer: Your dealer’s inability to diagnose the issue is not unusual, as is their apparent failure to involve the manufacturer early on …
Like other modern vehicles, Subarus are equipped with an event data recorder (EDR), separate from the engine control module (ECM or sometimes ECU), that is supposed to capture situations in which a customer pressed on the brake yet the vehicle continued to accelerate. Subaru Canada has regional technicians with the training and electronic equipment to retrieve those codes, as does Transport Canada for most brands. This equipment and capability rarely extend to the car dealer, whose diagnostic equipment focusses on powertrain performance issues and emissions compliance, but not performance of the airbags, seat belt tensioners, or brake and throttle in the moments prior to a collision. All modern passenger vehicles record some information of this nature. However, dealers will not necessarily flag an event for their manufacturer to investigate; instead, it’s the unhappy vehicle owner who escalates their complaint to head office in an attempt to obtain those resources.

I reviewed your situation with Eli Melnick, an engineer and defect investigator for insurance claims who provided the following perspective on your situation:

I feel for this guy but, crazy as it sounds, he’ll have to collide with something to trigger an event and enable evidence data to be downloaded. The dealer is trying but typically they won’t do anything unless they can experience the problem. Dealers cannot access the EDR. Even some Canadian importers once had to send the encrypted files overseas to head office. We can do it using an aftermarket tool and training. You need to know how to interpret the data—it’s pretty cryptic. In certain late model cars, we can even download video images from the front camera.

Eli Melnick

If you haven’t already done so, you should report the incidents to Transport Canada, which will record your complaint …

Most auto insurance companies are not interested in this sort of fact-finding unless the incident resulted in a large property loss or significant personal injuries. In that case, they may be interested in discovering the cause of the alleged event as a precursor to making a claim from the automaker. For damage limited to the vehicle, it’s usually cheaper and more expedient for them to consider the incident an at-fault claim by the owner. The vehicle owner, who may be desperate for clarity in the days after an unintended acceleration event like yours, usually has little influence.

(Here are some possible causes of sudden acceleration:)

Floor-mat interference with the pedals

… The first recommendation when having experienced a sudden unintended acceleration incident is to check that there is only one mat in the driver’s footwell and that you have not placed a winter mat over the carpeted mat delivered with the car, which can increase the likelihood of interfering with the pedals.

Furthermore, original equipment winter mats from many automakers frequently have a retention system, usually one or two eyelets or clips to prevent the mat from sliding forward and interfering with the pedals; that’s a failsafe most aftermarket mats, which are designed to fit a variety of vehicles, don’t feature. And many original-equipment mats are now shorter than the aftermarket variety.

Pedal error

The most common sudden acceleration complaint involves one event, in which the driver just sat down inside the vehicle or had just rotated their body to reverse the car … Incidents are usually reported in parking lots, driveways, garage entrances and car washes. From the driver’s perspective, it feels like the brake pedal sank to the floor (as if the brakes failed), or that the pedal is not responding. The likely cause is applying the wrong pedal. It happens more often than most people think, and it doesn’t take much misalignment of the torso or legs to occur …

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