Don't do what Piranha states. The codes will not be erased by disconnecting the battery you will only add more sensors off line. If you have DTC's stored they can be erased with a code reader. If the car has had the battery disconnected or gone dead this is where the problem is. If you have hard codes stored then the action required to fix them needs to be done. IE bad oxygen sensor then it needs to be replaced. Sensors off line like what I have are battery and run time related.