My car won't start without a jump, when I give it a jump, it runs great, but when I leave it off for long periods, the power drains out completely, so I thought it was probably the alternator, but went to the mechanic and was told its fine, the obd code says its cat converters, and mechanic asks for about $2000, what can I do?

Converters have nothing to do with needing to jump start a car to get it running. Your battery is bad or you have a slow drain on your battery. It actually sounds like it has a dead cell.

The converters would be a whole different issue.

When I have that problem Its usually the solenoid or a short.

It is not the cat converter, it has nothing to do with car not starting.

As noted, it is a sign that the battery is going dead, my first thought is a bad battery or a drain on the battery.

Second thought, do not ever take your car back to that other mechanic.

3 things most common would be a bad battery, battery not charging, or a drain.
Charge battery, read voltage, start car, if voltage does not increase, it is not charging.

An DC Amprobe, or ampmeter between the battery and removed battery cable, will show the amount of drain, have doors closed.

I've had similar problems with old cars. I would test the battery by removing the cables for a day or two, if it still holds a charge and the car will start after reconnecting them, then you have a slow drain on it.

If that were the case I would buy a set of quick disconnects for $6 and just unplug my battery when I parked it for more than a few hours.

If leaving it unhooked for a day or two results in it still not being able to start, then the battery no longer holds a charge. Time for a new battery. However, having a slow drain on a battery, even a new one can significantly shorten its life, so if it were still experiencing the problem, back to the quick disconnects.

This is not a fix, but it narrows down the problem and provides a work around.

If it's a newer car, disconnecting the battery for any length of time will reset the ECU but the biggest problem will also be your radio may lock out where you need a code to enter order to use it again.