* Transmission Problems: This is the most probable cause. The issue could stem from several internal transmission components:
* Worn or Damaged Synchronizers: These components allow for smooth gear changes. Wear or damage prevents engagement of 3rd and 4th gear.
* Broken or Worn Gears: A broken or severely worn gear in 3rd or 4th will prevent shifting into those gears.
* Shifting Forks or Rails: These components physically move the gears. Bent, broken, or worn forks or rails will prevent the gears from engaging.
* Low Transmission Fluid: Low fluid level leads to insufficient lubrication, causing damage to internal components and preventing shifting. Check your fluid level immediately – it should be within the proper range on the dipstick when the engine is running. *If it's low, investigate the reason – a leak is possible*.
* Clogged or Damaged Filters: A severely clogged filter can restrict fluid flow, negatively impacting transmission performance.
* Internal Valve Body Problems: The valve body controls the flow of fluid to the clutches and bands. Problems here can affect shifting.
* Shift Linkage Issues: While less likely to completely eliminate 3rd and 4th, problems with the linkage connecting the shifter to the transmission can cause missed shifts or difficulty engaging certain gears. Inspect for broken or disconnected linkages, bent shift levers, or worn bushings.
* Control Cable Problems (Automatic): If it's an automatic transmission, the control cable that regulates gear selection could be broken or damaged.
What to Do:
1. Check the Transmission Fluid: This is the first and most important step. Low or dirty fluid suggests a serious problem requiring attention.
2. Listen for Noises: Unusual noises (grinding, whining, etc.) during attempts to shift into 3rd or 4th give clues about the problem's location and severity.
3. Professional Diagnosis: The complexity of automatic transmissions strongly suggests seeking a professional diagnosis from a qualified mechanic specializing in transmissions. They have the tools and experience to properly assess the issue and recommend the necessary repairs. Attempting to fix this yourself without experience is likely to cause more damage or injury.
Ignoring the problem will almost certainly lead to more significant (and expensive) damage to your transmission. Get it checked promptly.
