1. Safety First:
* Park the car on a level surface and engage the parking brake firmly.
* Never work under a car supported only by a jack. Use jack stands for safety.
2. Initial Checks (Easy and Safe):
* Fluid Level and Condition: Check the automatic transmission fluid (ATF) level using the dipstick (check the engine's operating temperature first as specified in the owner's manual). Is the fluid low? Is it dark, burnt, or smells burnt? Low fluid or severely contaminated fluid indicates a significant problem.
* Battery Voltage: A weak battery can affect the transmission's operation, especially in electronic control systems. Test the battery voltage.
* Selector Lever: Ensure the gear selector is moving freely and engaging properly. If it feels stiff or binds, there may be a linkage problem.
* Check Engine Light (CEL): Is the CEL illuminated? If so, use an OBD-II scanner to retrieve the diagnostic trouble codes (DTCs). These codes can pinpoint potential issues. Auto parts stores often provide free code reading services.
3. Intermediate Checks (May Require Some Mechanical Knowledge):
* Transmission Linkage: Visually inspect the linkage connecting the gear selector to the transmission. Look for broken or bent components, loose connections, or binding points.
* Fluid Lines: Examine the transmission fluid lines for leaks or damage.
* Solenoids (If comfortable): If you have some mechanical experience and access to a multimeter, you can test the transmission solenoids. These electrical components control the shifting process. A malfunctioning solenoid can prevent shifting. This will require consulting a wiring diagram specific to your vehicle.
* Shift Cables (Manual Transmissions): If it's a manual transmission, inspect the shift cables for stretching or damage.
4. Advanced Diagnostics (Requires Specialized Tools and Knowledge):
* Pressure Testing: A transmission pressure test measures the hydraulic pressure within the transmission. Low or inconsistent pressure indicates internal problems. This requires specialized tools and knowledge.
* Scanning Tool with Transmission Data: Advanced scan tools can provide real-time data from the transmission control module (TCM), which is the computer controlling the transmission's operation. This can help isolate faulty sensors or electrical components.
* Road Test (With Caution): If the car can be moved safely, a road test can help determine if the transmission is slipping (a common sign of internal wear), or if the problem only occurs under certain conditions.
Possible Causes (based on your description of "not shifting"):
* Low or Contaminated Transmission Fluid: This is the most common and easiest to check.
* Faulty Solenoids or Sensors: Electrical components are prone to failure.
* Internal Transmission Problems: This is the worst-case scenario and usually requires a transmission rebuild or replacement. Problems can range from worn clutches and bands to a failure of internal components.
* Transmission Control Module (TCM) Failure: The TCM is the brain of the transmission; a failure requires replacement or reprogramming.
* Electrical Issues: Wiring harness problems, faulty connectors, or even a blown fuse can prevent proper shifting.
Disclaimer: This information is for informational purposes only. Attempting repairs without proper training and tools can lead to further damage or injury. If you are not comfortable performing these checks, take your vehicle to a qualified mechanic specializing in automatic transmissions. They have the tools and expertise to properly diagnose and repair the issue safely and efficiently.
