To quote my manual:-
Engine Warning Light
Illuminates when engine is switched on. It should extinguish as soon as the engine starts.
If it illuminates with the engine running, it indicates a malfunction. Have this checked by a Ford Dealer as soon as possible.
If it flashes while driving, reduce the vehicle speed immediately. If it continues to flash, avoid heavy acceleration and high engine speeds, and have your vehicle checked by a Ford Dealer immediately.
...So basically tells you nothing other than something's wrong but we want to rip you off and probably not find out what's wrong anyway.
In my opinion, I'd check to make sure you have put the leads back on the right plugs. Hard to get them wrong as they are usually cut to length so would be hard to put on wrong plug but you may have the leads wrong at the coil pack if you took them off.