* Shift Linkage Problem: This is the most probable cause. Something may have bound up, broken, or become dislodged in the mechanical linkage that connects the gear shifter to the transmission. This could be a broken cable, a bent or disconnected rod, or even something as simple as debris interfering with movement. This is something you (or a mechanic) can visually inspect.
* Transmission Shift Interlock Solenoid (or similar): Many automatic transmissions have a solenoid or other electrical component that prevents the shifter from moving into gear unless certain conditions are met (e.g., the brake pedal is pressed). Since only *half* the electrical system is working, there may be a problem with this specific solenoid or its associated wiring. A faulty neutral safety switch is also a common suspect here; it's a safety switch that prevents the car from starting unless the gear selector is in Park or Neutral.
* Low Transmission Fluid: While less likely to completely prevent shifter movement, extremely low transmission fluid can cause problems with shifting and could possibly contribute to the issue. Check your transmission fluid level; it should be at the "full" mark when the engine is running and warmed up (consult your owner's manual).
* Electrical Problem (Beyond the Interlock): The fact that only half your electrical system is working suggests a more serious electrical fault, perhaps a faulty alternator, a failing ground connection, or a problem within the main wiring harness. This could impact the transmission shift interlock, or other crucial components. This would require a more in-depth electrical diagnosis.
What to do:
1. Check the Transmission Fluid: This is the quickest and easiest check.
2. Visually Inspect the Shifter Linkage: Look under the hood and in the area around the shifter for any obvious damage, disconnections, or obstructions to the linkage.
3. Check Fuses: If you haven't already, check all fuses, especially those related to the transmission and other electrical systems.
4. Have it Towed: Attempting to force the shifter could cause further damage. It's best to have your Geo Prizm towed to a mechanic or repair shop for diagnosis and repair.
Trying to diagnose this yourself without experience could lead to further complications. A professional mechanic will have the tools and knowledge to accurately pinpoint the cause.
