* Low Transmission Fluid: This is the *first* thing to check. Low fluid can cause a multitude of transmission problems, including the inability to engage gears. Check the transmission fluid level (using the dipstick while the engine is running and warmed up) and condition (it should be reddish-pink and clear, not brown or burnt smelling). Low fluid may indicate a leak somewhere in the system that needs repair.
* Transmission Problems: This is a broad category, but the most likely culprits include:
* Torque Converter Issues: The torque converter connects the engine to the transmission. Problems here can prevent power transfer to the wheels.
* Solenoids or Valves: These electrical components control fluid flow within the transmission. A faulty solenoid or valve can prevent gear selection.
* Internal Transmission Damage: Worn gears, clutches, or other internal components can cause the transmission to fail to engage. This is usually the most expensive repair.
* Shift Linkage/Cable Issues (Less Likely): While less common on automatics, a problem with the linkage or cable that connects the shifter to the transmission could be the cause.
* Other Potential Issues (Less Likely):
* Engine Issues (Rare): While you say it starts fine, a very weak engine might not have enough power to overcome a minor transmission problem.
* Computer/Sensor Problems: A faulty sensor or a problem with the transmission control module (TCM) could be preventing proper gear selection. This is less likely if the transmission behaved properly for a while.
Troubleshooting Steps:
1. Check the Transmission Fluid: As mentioned above, this is the absolute first step. Top it off if low (use the correct type of ATF fluid for your Rodeo). However, if it's low, you still need to find the source of the leak.
2. Listen for Unusual Noises: When trying to shift, listen carefully for any unusual grinding, whining, or clunking noises coming from the transmission area. These can provide clues about the problem.
3. Check the Transmission Fluid Condition: If the fluid is dark, burnt, or smells bad, it indicates internal transmission problems that require professional attention.
4. Have it Professionally Diagnosed: If the fluid level is okay and you hear unusual noises or suspect internal transmission damage, it's best to take the vehicle to a qualified transmission specialist or mechanic. They have the tools and expertise to diagnose the issue properly.
Ignoring a transmission problem will likely lead to more significant and costly repairs later. Address the issue promptly.
