If your car is idling roughly, it could be due to a problem with the exhaust system, the engine cooling fans, or something else entirely. If the problem is with the exhaust system, it could be that the catalytic converter needs to be replaced. If the problem is with the engine cooling fans, it could be that they are not coming on when they should. Alternatively, if something else entirely is causing the problem, it could be a number of things. To diagnose the issue, you would need to take it to a mechanic and have them take a look at it.