Gearboxes, more commonly known as transmissions, can be a very expensive part to replace. To prevent damage to your transmission, or to avoid buying a car with existing problems, you should look for signs that indicate transmission failure. Since some transmissions operate better when they are cold, but show issues when heated up, it is always best to warm up the vehicle before attempting to diagnose a transmission issue.
Low Fluid Levels
Pull out the transmission dipstick to check the fluid levels. The transmission dipstick is usually located behind the oil dipstick but doesn't stick up as high. Wipe the dipstick with a cloth, reinsert it, then pull it out to check the level. If the dipstick shows the level is low, this could be an indicator that you have a fluid leak in your transmission. When you test the levels, you can also test the quality of the fluid by dripping it onto a white piece of paper. The oil should be clear and not have any black flakes in it. Smell the fluid. If you detect a hint of a burnt smell, this is also a bad sign that could indicate transmission issues.
One of the signs of transmission problems is delayed engagement when you shift gears, for example shifting from Drive to Reverse. Noticing delayed engagement is easiest if you let the car sit in the Park position for a while and then try to switch gears. Try starting the engine in Park and then, with your foot on the brake, switch into the Drive position. The car should feel like it wants to move forward, but the transition should be smooth. If there is a delay between when you switch gears and when the transmission kicks in, the transmission might be wearing out.
Noise or Jerky Movements When Shifting Gears
When the transmission shifts you should notice a slight change in the tone of the engine or note a change in the rpm. However, with transmissions that are extremely worn, shifting will not be smooth --- especially between first and second gear. If you notice any loud noises or if the car makes jerky movements when switching gears, this is a good indication of transmission failure.
A slipping transmission is the result of a transmission that has become extremely worn. You can test to see if this is an issue by driving your car, accelerating and checking the rpm reading. If you are pressing the accelerator and remain at a constant speed but the rpm continues to rise this is a sign of a slipped transmission.
Check Engine Light
If the check engine light comes on, this is an indication of problems in your engine that could be caused by your transmission. Your transmission might not always be at fault but you should still make sure to heed any warnings that appear on your car's dashboard. Postponing advised car maintenance could worsen the issue.