2004 Ford Freestar SEL sputtering, check engine light flickering.

My husband just called and said that our 2004 Ford Freestar SEL started sputtering while driving, and then the check engine light is blinking, not solid. What could this be? I hate to have to shell out a ton of money at the dealership. I just had my oil changed, cabin and engine air filters replaced, and 2 front tires replaced.

Engine lamp will come on if idle is too low / fuel mixture is too lean or too rich.
A faulty spark plug will cause this also.