* Low Transmission Fluid: This is the most common and often easiest to fix. Low fluid level means insufficient lubrication and pressure, preventing the transmission from engaging reverse. Check your transmission fluid level (consult your owner's manual for location and procedure). Low fluid often indicates a leak somewhere in the system, which also needs fixing.
* Faulty Transmission Fluid: Contaminated or degraded transmission fluid loses its lubricating and hydraulic properties, affecting the transmission's ability to engage gears. A fluid change is often necessary.
* Problems with the Reverse Clutch Pack: This is a set of friction plates and steels within the transmission that engage reverse gear. Wear, damage, or failure of this pack can prevent reverse from engaging. This typically requires a transmission repair or rebuild.
* Worn or Damaged Planetary Gearset: The planetary gearset is a complex assembly of gears that allows for different gear ratios. Damage here can prevent reverse engagement. This is a significant repair.
* Broken or Damaged Shift Linkage or Cables: This is less common in modern electronically controlled transmissions but still possible in older vehicles with mechanical linkages. A problem with the linkage or cables prevents the transmission from receiving the signal to engage reverse.
* Faulty Valve Body or Solenoids: The valve body controls the flow of transmission fluid. Malfunctioning solenoids (electromagnetic valves) or internal valve body problems can prevent reverse from engaging. This usually requires a transmission repair or replacement of the valve body.
* Broken or Stripped Reverse Gear: This is a severe problem within the transmission itself. A broken or stripped reverse gear will prevent it from functioning. Repair or replacement of the transmission is necessary.
* Electrical Problems (Modern Transmissions): In modern vehicles with electronic controls, issues with the transmission control module (TCM), wiring harness, or sensors can prevent reverse from engaging. Diagnosis requires specialized tools.
* Torque Converter Issues: While less likely to be the *sole* cause of a complete lack of reverse, problems with the torque converter can contribute to shifting problems, and could prevent reverse from engaging.
In short: The lack of reverse gear in an automatic transmission points to a significant problem. Do *not* attempt to drive the vehicle. Have it towed to a qualified mechanic or transmission specialist for diagnosis and repair. Driving with a transmission problem can cause further damage and lead to costly repairs.
