When auto transmissions go bad, there is a fairly limited range of options for dealing with the situation. These four choices are the ones that auto transmission services technicians regularly recommend to their customers.
Replacing with a New or Remanned System
This often sounds like the most appealing option, but the term "new" needs to be examined a little bit. Even with newer cars, manufacturers don't produce large surpluses of transmission units. This means there aren't a lot of genuinely brand new transmissions available for purchase.
Most newly installed systems are remanned. These are systems that have been remanufactured. That means they were sent back to the manufacturer due to defects, and technicians at one of the company's subdivisions or subcontractors picked through the buggy parts, replace them and tested the reassembled unit. There's nothing wrong with a reman, but be aware that it might not be as new as advertised.
Putting in a Used Part
Particularly when dealing with vehicles that are no longer in production or ones that are over 5 years old, installing a used transmission might be the best available option. These usually come from junked vehicles where the damage occurred on the other side of the car. For example, a salvage yard might not resell the transmission from a head-on collision, but one from a rear-end accident might be perfectly suitable. Many of these used transmissions are sold with limited warranties, but make sure you ask for one before you pay.
Sometimes the larger transmission unit is still in excellent shape and a simpler component might be having trouble. On electronic systems, this could be as simple as replacing a relay. Even on physically activated setups, it's not unusual for a component like a flywheel to get damaged. As long as the vehicle wasn't run too long in this condition, replacing the flywheel is a worthwhile solution.
A transmission rebuild is similar to using a reman, but the big difference is the work is done right there in the shop. In many cases, there are kits available for doing the rebuild on specific systems. In rare cases, individual, non-kit components may have to be purchased or machined.
A technician will drop the transmission out of the vehicle, open up the case, and swap out seals and other components that can wear down or crack. They'll button everything back up, put it in the vehicle and test it.