I have a 2007 ford mustang GT. The engine light came on and stays on. What's wrong?

I have over 172000 on the vehicle.

Hi simple the engine management has a problem so it turns the light on. So it needs to go to a ford dealer to have teh diagnostic tester plugged in. This will then reveal the code which has caused the issue. Failure to take it to a ford dealer usually results in the engine management system in a few miles switching into LIMP HOME MODE.

An engine light may come on for a lot of reasons.
Go to a shop and have it checked out

Have ford check it out (1 hour shop time) to diagnose the cause of the check engine light. They will give you options on what do to for your repair.