2001 Mercury Sable won't start
 

My sisters sable won't start after sitting a few hours. It cranks over normal but won't fire. If you put a charger on it for 5-10 minutes it will start with no problem. The problem is intermittent but usually happens after the car has been sitting a few hours. Any guidance is greatly appreciated, thanks

If there is any type of corrosion or poor connection from the battery to the other parts of the electrical system there is not enough power available to feed the circuits and a lot of them will not function. You might want to start with checking the connections at the battery. They might look OK,but still offer a high enough resistance to prevent full power to get to the rest of the vehicle. Make sure they are clean and tight.

The cables have been cleaned, the battery has been replaced, and the alt tested bad and was replaced

What is the PATS (Passive Anti-Theft System) light doing? If you have the second key handy try that key in the meantime.