* Faulty transmission linkage: The mechanical linkage connecting the shifter to the transmission could be bent, broken, or misadjusted.
* Shifting mechanism failure: Internal components within the transmission itself might be damaged or worn, causing it to shift incorrectly.
* Electrical problem: The automatic transmission relies on electrical components for control. A faulty sensor, wiring harness issue, or problem with the transmission control module (TCM) could lead to erratic shifting.
* Low transmission fluid: While less likely to cause a direct shift into reverse while driving forward, critically low fluid could cause significant internal damage and erratic behavior.
* Internal transmission damage: Serious internal damage within the transmission could be causing the problem.
It is extremely dangerous to drive a car with this issue. Shifting unexpectedly into reverse at speed could lead to a serious accident. The vehicle needs to be towed to a qualified mechanic specializing in automatic transmissions for diagnosis and repair. Attempting to diagnose or fix this yourself is not recommended unless you have significant experience working on automatic transmissions. Improper repair could worsen the damage.
