* Faulty or improperly adjusted automatic transmission linkage: This is the most common cause. If the linkage isn't properly adjusted or is damaged, it might not fully engage the parking pawl (the mechanism that locks the transmission in Park). Even a slight disengagement can allow the car to creep forward. This is particularly likely if the car is on a slight incline.
* Worn or damaged transmission components: Internal transmission problems, such as worn bands or a slipping torque converter, can sometimes lead to the car unexpectedly moving forward upon starting. This is less likely than a linkage issue, but possible.
* Parking brake not fully engaged: If the parking brake isn't fully applied, the car might roll slightly forward when the engine starts. This is more obvious on a slope.
* Shift lever not fully in Park (P): Make absolutely sure the shift lever is securely in the Park position. A slightly loose or misaligned lever can sometimes cause this.
* Low transmission fluid: While less likely to cause a *jump*, low transmission fluid can contribute to slipping and erratic behavior, potentially leading to unexpected movement.
* Faulty Neutral Safety Switch (NSS): This switch prevents the car from starting unless the gear selector is in Park or Neutral. A faulty NSS might allow the engine to crank even if the transmission isn't fully in Park.
* Software glitch (modern cars): In some modern vehicles with complex electronic systems, a software glitch could theoretically cause this issue. This is rarer.
What to do:
If your car is jumping forward when starting, do not ignore it. This is a safety hazard. Have a qualified mechanic inspect the vehicle immediately to diagnose and repair the problem. Driving a car with this issue could lead to an accident.
