Battery died and a new one installed on 2004 ford freestar. The check engine light came and stayed on?

The check engine light is an indicator that the computer has detected a fault in one or more of the electronic components. Use a code scanner to figure out what is wrong.

Car/Engine codes can be read with a plug-in diagnostic tool that your local auto parts dealer will be happy to read for you. You could have many different problems major or very minor that effect the performance and health of your engine. It could be a simple air flow problem, maybe a filter, a misfire problem, or a fuel or oxygen sensor malfunction.