On my 1994 F-150 with a 351 engine and E4OD transmission the check engine light is constantly flashing. I checked both the motor oil level and the coolant level and they are both full. I drove it 20 miles to town and 20 miles back home and everything worked flawlessly. Both the oil pressure gauge and the temperature gauge read normal.

What are some of the things that can cause the check engine light to flash?

 

Everything EFI related, you need to pull codes to see what is up.

 

The ONLY thing that can make the check engine light flash is misfire. If flashes to warn you that misfire is damaging the catalytic converter.

 

Here's a link for you:
Fuel Injection Technical Library » How To Run a Self-Test