Most Likely Causes:
* Low Transmission Fluid: This is the first thing you should check. Low fluid can cause all sorts of shifting problems, including the inability to upshift. Check the transmission fluid level with the engine running and warmed up (following the procedure in your owner's manual). If it's low, you'll need to add fluid, but be aware that low fluid is often a symptom of a bigger problem (leak). Do not overfill.
* Faulty Transmission Solenoid(s): These electromagnets control the shifting within the transmission. A malfunctioning solenoid can prevent the transmission from shifting correctly. This is a relatively common and sometimes repairable issue.
* Transmission Valve Body Problems: The valve body houses the solenoids and other components that control fluid flow. Worn or damaged components inside the valve body can cause a variety of shifting problems. Repair usually involves replacing the valve body or rebuilding it.
* Worn Clutch Packs or Bands: These components inside the transmission physically engage and disengage to create the gear changes. Wear and tear, especially with age and mileage, can prevent proper upshifting. This is a more serious and expensive repair.
Less Likely (but still possible) Causes:
* Transmission Computer (TCM) Issues: The TCM controls the transmission's operation. A faulty TCM can lead to incorrect shifting patterns. Diagnosis requires specialized tools.
* Throttle Position Sensor (TPS) Problem: While less common for this specific symptom, a faulty TPS can send incorrect information to the TCM, affecting shifting.
* Speed Sensor Issues: A malfunctioning speed sensor can prevent proper gear selection.
What to do:
1. Check the transmission fluid level immediately. This is the easiest and cheapest check, and it could solve the problem.
2. Have the transmission fluid and filter changed. Even if the level is OK, old, dirty fluid can cause problems.
3. Take it to a reputable transmission specialist or mechanic. Diagnosing transmission problems requires expertise and specialized equipment. They can perform a proper diagnosis to pinpoint the exact cause. Trying to fix a transmission yourself without the right knowledge and tools can lead to further damage and increased costs.
Important Note: Driving a car with a transmission that won't shift up properly can cause further damage to the transmission. Minimize driving until you have it diagnosed and repaired. Driving only long enough to get it to the mechanic is recommended.
